Issue #195: refactor backend specific code out of controllers (conservancy/kallithea)

[Dominik Ruf]

New issue 195: refactor backend specific code out of controllers
https://bitbucket.org/conservancy/kallithea/issues/195/refactor-backend-specific-code-out-of

Dominik Ruf:

There is quite some backend specific code in the controllers (e.g. controllers/pullrequests.py)
Where possible this should be refactor in to lib.vcs.backends.xxx