Add support for rich references to referables

Description

After looking at current GitLab CI Plan issue https://gitlab.com/gitlab-org/gitlab-ce/issues/22445, I thought about having rich references to referables (issues, merge requests, snippets, labels etc).

Proposal

Using references and cross references like:

gitlab-org/gitlab-ce#12345+, gitlab-org/gitlab-ce!12345++, #12345+++

This would work as usual, but it would support additional + at the end. Depending of number of pluses (verbosity level) it could unfold to something like:

1. #1234+

   #1234 Some awesome feature proposal issue title

2. #1234++

   #1234 Some awesome feature proposal issue title ~CI ~"feature proposal"

3. #1234+++

   #1234 Some awesome feature proposal issue title ~CI ~"feature proposal" Author: (avatar, username) MRs: (number of MRs referenced from issue)

This would obviously require some frontend work to provide great rich-reference-widget.

It should support probably all types of referables, but issues and merge requests would be most important.

For MRs for !12345+++ we can show:

!12345 Some awesome merge request ~CI ~backstage Author: (avatar, username) Commits: (number of commits) [ or Versions: (number of MR versions)]

I think this can greatly improve context of the discussion without the need of clicking issues and merge requests, to see some of most important details.

What do you think @JobV @DouweM @smcgivern?