Add logic to detect wiki page deletion

[rsyring]

GH's event system doesn't trigger a gollum event for wiki page deletes. So, currently, a page can only be permanently deleted from the source repo.

I think we could do some minimal state tracking on the files such that we could detect when a file exists in the docs repo but not the wiki if the file is new to the docs repo and should be synced or was previously synced to the wiki and its absence means we should delete it rather than sync it.