Add email validity check (#13475)

* Improve error feedback for duplicate deploy keys

Instead of a generic HTTP 500 error page, a flash message is rendered
with the deploy key page template so inform the user that a key with the
intended title already exists.

* API returns 422 error when key with name exists

* Add email validity checking

Add email validity checking for the following routes:
[Web interface]
1. User registration
2. User creation by admin
3. Adding an email through user settings
[API]
1. POST /admin/users
2. PATCH /admin/users/:username
3. POST /user/emails

* Add further tests

* Add signup email tests

* Add email validity check for linking existing account

* Address PR comments

* Remove unneeded DB session

* Move email check to updateUser

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

models/user_test.go

@@ -329,6 +329,21 @@ func TestCreateUser(t *testing.T) {
 	assert.NoError(t, DeleteUser(user))
 }
 
+func TestCreateUserInvalidEmail(t *testing.T) {
+	user := &User{
+		Name:               "GiteaBot",
+		Email:              "GiteaBot@gitea.io\r\n",
+		Passwd:             ";p['////..-++']",
+		IsAdmin:            false,
+		Theme:              setting.UI.DefaultTheme,
+		MustChangePassword: false,
+	}
+
+	err := CreateUser(user)
+	assert.Error(t, err)
+	assert.True(t, IsErrEmailInvalid(err))
+}
+
 func TestCreateUser_Issue5882(t *testing.T) {
 
 	// Init settings