Currently this works by loading the HAML partials via XHR. While this is
not the nicest setup it _is_ the easiest setup using the tools we
currently have.
Loading this data asynchronously doesn't make loading the related
MRs/branches itself faster, it merely ensures that loading the issue
itself is not slowed down.
Fixesgitlab-org/gitlab-ce#14949
1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
2. Also, refactored a bit the Issuable helpers into a new IssuablesHelper
module.
3. Added acceptance specs for the sidebar counter.
Create a new shared module for common issue/merge request behavior,
use `expect` syntax instead of `should`, and avoid `visible: false` in
the `have_css` matcher.