Custom icons for OAuth sources (#14161)

* Add Icon URL to Backend

* Template for Icon URL

* Localization & Edit Icon URL

* Improve Custom Icon URL

* Removed not working else

Co-authored-by: 6543 <6543@obermui.de>

models/login_source.go

@@ -131,6 +131,7 @@ type OAuth2Config struct {
 	ClientSecret                  string
 	OpenIDConnectAutoDiscoveryURL string
 	CustomURLMapping              *oauth2.CustomURLMapping
+	IconURL                       string
 }
 
 // FromDB fills up an OAuth2Config from serialized format.