This PR adds basic repository LFS management UI including the ability to find all possible pointers within the repository. Locks are not managed at present but would be addable through some simple additions.

* Add basic repository lfs management
* add auto-associate function
* Add functionality to find commits with this lfs file
* Add link to find commits on the lfs file view
* Adjust commit view to state the likely branch causing the commit
* Only read Oid from database

* extract actions on new pull request from models to pulls service

* improve code

* move code.gitea.io/gitea/modules/pull to code.gitea.io/gitea/services/pull

* fix fmt

* Rename pulls.go to pull.go

No need to convert to the same type.

On merge we walk the merge history and ensure that all lfs objects pointed to in
the history are added to the base repository. This switches from relying on having git-lfs installed on the server, (and in fact .gitattributes being correctly installed.)