Store both the project and identifier of extracted references.  This
prevents `ReferenceExtractor` from returning objects in the wrong
project for cross-project references.

The cross-project reference feature broke the ReferenceExtractor class;
this fixes it.

Any mention of Issues, MergeRequests, or Commits via GitLab-flavored markdown
references in descriptions, titles, or attached Notes creates a back-reference
Note that links to the original referencer. Furthermore, pushing commits with
commit messages that match a (configurable) regexp to a project's default
branch will close any issues mentioned by GFM in the matched closing phrase.
If accepting a merge request would close any Issues in this way, a banner is
appended to the merge request's main panel to indicate this.