Support LFS for wiki uploads

Wiki images are currently stored as uploads outside the repository rather than objects in the Git repo. Once this is fixed by https://gitlab.com/gitlab-org/gitlab-ce/issues/33475 we should add support for Git LFS so that the repo doesn't become filled with binary data.

Changes to Gollumn may be required to support this.

Proposal

• If the wiki repository contains a .gitattributes file it should be respected when uploading files, as previously implemented for normal repositories https://gitlab.com/gitlab-org/gitlab-ce/issues/29876
• New wikis should be created with .gitattributes by default

/uploads/* filter=lfs diff=lfs merge=lfs -text

This should be for new uploads to the Wiki, we can tackle migration in a later iteration.

Links

https://gitlab.com/gitlab-org/gitlab-ce/issues/33475