Sync project dev info to Jira after adding a namespace subscription

After adding a namespace to subscribe to, we sync any new updates for projects under that namespace (https://gitlab.com/gitlab-org/gitlab-ee/issues/9643).

We should also do an initial sync of the existing commits, branches, and merge requests.

Once we have this full-sync capability, we can also consider enabling manually triggering this sync through the UI https://gitlab.com/gitlab-org/gitlab-ee/issues/13074

When we implement this we should take into consideration the possible N+1 DB queries / Gitaly calls when we send multiple commits / branches / MRs to Jira.