Change ajax and cronjob endpoints - Fixes #767

This commit changes the ajax and cronjob endpoints:
/src/Backend/Ajax.php > /backend/ajax
/src/Frontend/Ajax.php > /frontend/ajax
/src/Backend/Cronjob.php > /backend/cronjob.

This fixes some problems with servers interpreting the .php extension as
a file. This way, the request doesn't get rewritten to the index.php
frontend controller.
I think we should look into other web servers as well.