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>

options/locale/locale_en-US.ini

@@ -2203,6 +2203,7 @@ auths.enable_tls = Enable TLS Encryption
 auths.skip_tls_verify = Skip TLS Verify
 auths.pam_service_name = PAM Service Name
 auths.oauth2_provider = OAuth2 Provider
+auths.oauth2_icon_url = Icon URL
 auths.oauth2_clientID = Client ID (Key)
 auths.oauth2_clientSecret = Client Secret
 auths.openIdConnectAutoDiscoveryURL = OpenID Connect Auto Discovery URL