Split clients/repository for posts/media
It didn't really make sense in the past that we used the same GitLabClient
for the use of posts and media publishing, not just because the plan is to move
away from a GitLab-backed repo for media storage, but also because it makes
sense to keep them separate.
We can extract common functionality into a new AbstractGitLabClient and
create an interface for PostClient and MediaClient, so it's not specific to
GitLab.
Closes #257 (closed).
Edited by Jamie Tanna