After OAUTH, SAML auth completion, Redirect to App custom url scheme with token data as query params. (#16447)

* Added redirection after Auth complete

* Fixed gofmt

* Handling error while parsing the url, added util function to check for a valid mobile redirect url

* Added check for custom scheme url validation

* Added test to verify custom schele url

* Added mobile message screens

* Translation strings for mobile screens

* Added mobile specific screens for success and error

* Added error logs and changed variable name for consistency with oauth.go

* i18n fix

* Reusing assigned variable instead of map

* Added AppCustomUrlScheme property

* Code refactor and removed dependency from cookies to build the final url

* Changed util function

* Updated util test

* Fixed go lint

* Code refactor and unsused code removed

* Code refactor

* simplified boolean checks

* Added support of whitelist of appCustomURLSchemes

* Changed i18 en

* Fixed validating redirecturl for web

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/config.go

@@ -237,6 +237,10 @@ const (
 	LOCAL_MODE_SOCKET_PATH = "/var/tmp/mattermost_local.socket"
 )
 
+func GetDefaultAppCustomURLSchemes() []string {
+	return []string{"mmauth://", "mmauthbeta://"}
+}
+
 var ServerTLSSupportedCiphers = map[string]uint16{
 	"TLS_RSA_WITH_RC4_128_SHA":                tls.TLS_RSA_WITH_RC4_128_SHA,
 	"TLS_RSA_WITH_3DES_EDE_CBC_SHA":           tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA,
@@ -2427,9 +2431,10 @@ func (s *SamlSettings) SetDefaults() {
 }
 
 type NativeAppSettings struct {
-	AppDownloadLink        *string `access:"site,write_restrictable,cloud_restrictable"`
-	AndroidAppDownloadLink *string `access:"site,write_restrictable,cloud_restrictable"`
-	IosAppDownloadLink     *string `access:"site,write_restrictable,cloud_restrictable"`
+	AppCustomURLSchemes    []string `access:"site,write_restrictable,cloud_restrictable"`
+	AppDownloadLink        *string  `access:"site,write_restrictable,cloud_restrictable"`
+	AndroidAppDownloadLink *string  `access:"site,write_restrictable,cloud_restrictable"`
+	IosAppDownloadLink     *string  `access:"site,write_restrictable,cloud_restrictable"`
 }
 
 func (s *NativeAppSettings) SetDefaults() {
@@ -2444,6 +2449,10 @@ func (s *NativeAppSettings) SetDefaults() {
 	if s.IosAppDownloadLink == nil {
 		s.IosAppDownloadLink = NewString(NATIVEAPP_SETTINGS_DEFAULT_IOS_APP_DOWNLOAD_LINK)
 	}
+
+	if s.AppCustomURLSchemes == nil {
+		s.AppCustomURLSchemes = GetDefaultAppCustomURLSchemes()
+	}
 }
 
 type ElasticsearchSettings struct {