Oauth2 consumer (#679)

* initial stuff for oauth2 login, fails on:
* login button on the signIn page to start the OAuth2 flow and a callback for each provider
Only GitHub is implemented for now
* show login button only when the OAuth2 consumer is configured (and activated)
* create macaron group for oauth2 urls
* prevent net/http in modules (other then oauth2)
* use a new data sessions oauth2 folder for storing the oauth2 session data
* add missing 2FA when this is enabled on the user
* add password option for OAuth2 user , for use with git over http and login to the GUI
* add tip for registering a GitHub OAuth application
* at startup of Gitea register all configured providers and also on adding/deleting of new providers
* custom handling of errors in oauth2 request init + show better tip
* add ExternalLoginUser model and migration script to add it to database
* link a external account to an existing account (still need to handle wrong login and signup) and remove if user is removed
* remove the linked external account from the user his settings
* if user is unknown we allow him to register a new account or link it to some existing account
* sign up with button on signin page (als change OAuth2Provider structure so we can store basic stuff about providers)

* from gorilla/sessions docs:
"Important Note: If you aren't using gorilla/mux, you need to wrap your handlers with context.ClearHandler as or else you will leak memory!"
(we're using gorilla/sessions for storing oauth2 sessions)

* use updated goth lib that now supports getting the OAuth2 user if the AccessToken is still valid instead of re-authenticating (prevent flooding the OAuth2 provider)

modules/auth/oauth2/oauth2.go

@@ -0,0 +1,105 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package oauth2
+
+import (
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/log"
+	"github.com/gorilla/sessions"
+	"github.com/markbates/goth"
+	"github.com/markbates/goth/gothic"
+	"net/http"
+	"os"
+	"github.com/satori/go.uuid"
+	"path/filepath"
+	"github.com/markbates/goth/providers/github"
+)
+
+var (
+	sessionUsersStoreKey = "gitea-oauth2-sessions"
+	providerHeaderKey    = "gitea-oauth2-provider"
+)
+
+// Init initialize the setup of the OAuth2 library
+func Init() {
+	sessionDir := filepath.Join(setting.AppDataPath, "sessions", "oauth2")
+	if err := os.MkdirAll(sessionDir, 0700); err != nil {
+		log.Fatal(4, "Fail to create dir %s: %v", sessionDir, err)
+	}
+
+	gothic.Store = sessions.NewFilesystemStore(sessionDir, []byte(sessionUsersStoreKey))
+
+	gothic.SetState = func(req *http.Request) string {
+		return uuid.NewV4().String()
+	}
+
+	gothic.GetProviderName = func(req *http.Request) (string, error) {
+		return req.Header.Get(providerHeaderKey), nil
+	}
+
+}
+
+// Auth OAuth2 auth service
+func Auth(provider string, request *http.Request, response http.ResponseWriter) error {
+	// not sure if goth is thread safe (?) when using multiple providers
+	request.Header.Set(providerHeaderKey, provider)
+
+	// don't use the default gothic begin handler to prevent issues when some error occurs
+	// normally the gothic library will write some custom stuff to the response instead of our own nice error page
+	//gothic.BeginAuthHandler(response, request)
+
+	url, err := gothic.GetAuthURL(response, request)
+	if err == nil {
+		http.Redirect(response, request, url, http.StatusTemporaryRedirect)
+	}
+	return err
+}
+
+// ProviderCallback handles OAuth callback, resolve to a goth user and send back to original url
+// this will trigger a new authentication request, but because we save it in the session we can use that
+func ProviderCallback(provider string, request *http.Request, response http.ResponseWriter) (goth.User, error) {
+	// not sure if goth is thread safe (?) when using multiple providers
+	request.Header.Set(providerHeaderKey, provider)
+
+	user, err := gothic.CompleteUserAuth(response, request)
+	if err != nil {
+		return user, err
+	}
+
+	return user, nil
+}
+
+// RegisterProvider register a OAuth2 provider in goth lib
+func RegisterProvider(providerName, providerType, clientID, clientSecret string) {
+	provider := createProvider(providerName, providerType, clientID, clientSecret)
+
+	if provider != nil {
+		goth.UseProviders(provider)
+	}
+}
+
+// RemoveProvider removes the given OAuth2 provider from the goth lib
+func RemoveProvider(providerName string) {
+	delete(goth.GetProviders(), providerName)
+}
+
+// used to create different types of goth providers
+func createProvider(providerName, providerType, clientID, clientSecret string) goth.Provider {
+	callbackURL := setting.AppURL + "user/oauth2/" + providerName + "/callback"
+
+	var provider goth.Provider
+
+	switch providerType {
+	case "github":
+		provider = github.New(clientID, clientSecret, callbackURL, "user:email")
+	}
+
+	// always set the name if provider is created so we can support multiple setups of 1 provider
+	if provider != nil {
+		provider.SetName(providerName)
+	}
+
+	return provider
+}