Solve URL ambiguity between LFS and git repository file update paths

Per gitlab-workhorse#197 (comment 130790885)

Currently, there is an ambiguity with a request that looks like:

PUT https://gitlab.com/group/subgroup/tree/master/project.git/gitlab-lfs/objects/aaaaaaa/10

This is because the request may come from either an LFS client trying to create a new file, or (with a bit of trickery) the web UI, asking to update the file project.git/gitlab-lfs/objects/aaaaaaa/10 in a project called "tree".

To resolve this ambiguity, we select on content-type, but it would be better to move one of the two functions to an URL with no possibility of ambiguity.

One option is to move the LFS API to /api/lfs/<project-spec>/objects/:sha/:id. I like this one a lot :D. It could go to /<group>/<project>/-/gitlab-lfs/objects/:sha/:id instead, if we prefer that, but it's a lot harder to validate in workhorse, haproxy, etc, since we have to ensure that the /-/ component is the first in the URL, while the rest must be the URL termination.

Another option is to move the web edit UI, and all its functionality, to be behind a /-/ prefix in the same way. I like this much less, since it seems like a bigger breaking change - those URLs are scattered throughout our own issues as references, for instance.

cc @DouweM