Fetch commit comments of a repository

Note: There is not sorting for this api endpoint :(

cf #28
It's now possible to fetch labels and issues events if repo doesn't have issues
Remove the unused FetchUnfetchedCommits job
Manage commit comments in events/hooks

cf #28
Don't use mini "per_page" option if not fetched for one day
Add missing SearchReferenceCommitForCommitComment job

cf #28
Correct bug when fetching non-existent commit
Keep commits removed from PRs

When a pull request is rebased, the olds commits are not tied to this
pull request anymore. But if there were comments attached to theses
commits, we want to display them, so we must keep the relation issue-
commit.

Know, this relation is done not via s simple m2m but via a through
table, with a new attribute, 'deleted', that is True if the commit is
not anymore tied to the issue.

cf #28
Manage new IssueCommits through table in activity feeds

cf #28
Hide deleted commits on pr detail view

cf #28
Allow display of outdated commits on commits tab

cf #28
Display comments count on commits

cf #28
Add notif about commit comments in pr discuss tab

cf #28
Display commit comments in activity feed

cf #28
Add missing indexes
Merge branch 'feature/commit-comments' into develop

Fixed #28

Actually commit comments are not displayed and so there is no way to
answer them. There is just notifications about such comments, with the
commit displayed to see them on Github.

To resove all of this, #105 must be implemented.