[backend] enforce sha256 for newer rpm-md repositories
[api] refactor status controller
[api] refactor one more function
[api] srcmd5 should probably not be a class variable but an instance variable
[api] replace linked_packages with backend_packages

Also saves md5sum for all packages not just links
[activexml] do not retry failed connections

This was needed back in lighttd days
[api] add the possibility to subscribe to events

You can subscribe globally or on project/package leel if you want to
get notifications (not yet implemented obviously) for events either
for
- none (overwriting more global defaults)
- project maintainership
- package maintainership
- all
[webui] add basic functionality to edit user subscriptions