OAuth: clear refresh token when the server claim not to support oauth

Allow upgrade path when the server removes support for oauth
Relates: #5848 (comment)

We also need to force the account to commit the config to the disk,
otherwise we may not register we are no longer using owncloud and we
risk sending the password as the token to the token refresh API call

src/gui/creds/httpcredentialsgui.cpp

@@ -120,6 +120,7 @@ void HttpCredentialsGui::showDialog()
     bool ok = dialog.exec();
     if (ok) {
         _password = dialog.textValue();
+        _refreshToken.clear();
         _ready = true;
         persist();
     }

src/libsync/creds/httpcredentials.cpp

@@ -397,6 +397,7 @@ void HttpCredentials::persist()
 
     _account->setCredentialSetting(QLatin1String(userC), _user);
     _account->setCredentialSetting(QLatin1String(isOAuthC), isUsingOAuth());
+    _account->wantsAccountSaved(_account);
 
     // write cert
     WritePasswordJob *job = new WritePasswordJob(Theme::instance()->appName());