Feature/svn repo integration

This PR aims to re-integrate the (currently private) plugin [repository_authentication](https://github.com/finnlabs/openproject-repository_authentication) back into the OpenProject core.

This enabled everyone to have subversions repositories again (and might be extended for git support).
Also, this is a step to have an easier installation using packager.io (as cyril at packager.io is currently working on configuration scripts for svn integration - and he needs the parts provided in this PR).


See: https://www.openproject.org/work_packages/13898