Support rich references in links

Currently supports up to 1 rich reference verbosity level. Level 1
includes the referable title in the link text. The rich reference suffix
is '+'. More than 3 '+' characters are ingored.

So, `#123` becomes a link with text "123 The issue title".

- Modified issue regex to account for rich references verbosity
- Modified IssueReferenceFilter to add a data-rich-ref-verbosity attribute
- Added a post processing pipeline filter (RichReferenceFilter) to deal with
rich references
48 jobs for gkats/gitlab-ce-23177-support-plus-sign-for-referable-title in 59 minutes and 58 seconds (queued for 43 seconds)
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