Fix for #1307 (Oauth login not working)

- Changed blueprint reference in Backend from name to id, users can now again be found
- Depending on version of flask-dance the resultcode of authorized is set different to prevent redirect loop with newer versions(>1.3)
- Redirect to login page in case no user is linked to current oauth account
- Flash messages upon successfull login, successsfull linked account

cps/templates/user_edit.html

@@ -46,7 +46,7 @@
             {% endfor %}
         </select>
     </div>
-    {% if registered_oauth.keys()| length > 0 %}
+    {% if registered_oauth.keys()| length > 0 and not new_user %}
       {% for id, name in registered_oauth.items() %}
     <div class="form-group">
       <label>{{ name }} {{_('OAuth Settings')}}</label>