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.

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.

Edited May 28, 2019 by Kai Armstrong