This lets us know which repository the LFS pointer file is in, in order
to prune unreferenced LFS objects in the future.

Saving LFS objects is intended to now be supported for repositories
other than the regular 'project' repository; the extra repositories that
will also support LFS objects are 'wiki' and 'design'.

Because an LFS object's oid is determined by the data in the file, it
will be possible for a project to have the same LfsObject in each of the
three different repositories.

Which repository the LFS object lives in will now be
recorded in the has-and-belongs-to-many relation model
LfsObjectsProject. In the situation where the same blob is saved in, for
e.g., the 'project' and 'design' repositories, there will be two
LfsObjectsProject records created for each of the two repository_types.
This means the has_many relationships between lfs_objects and projects
now need a `distinct` scope to limit the records returned to
non-duplicates.

Lfs::FileTransformer will now create LfsObjectsProjects with
a repository_type saved. The DesignManagement::SaveDesignsService is the
first part of the app to pass the repository_type to this class
to have it saved.

There is a technical debt task to have all parts of the app that create
LFS objects save repository_type to LfsObjectsProject in future.

https://gitlab.com/gitlab-org/gitlab-ee/issues/9490 particular this
comment thread
https://gitlab.com/gitlab-org/gitlab-ee/issues/9490#note_155912249