Support mentioning epics

Requirements

• If there is a url to an epic in a textbox, when it is rendered, it will be rendered as an epic link, per the formatting here: https://gitlab.com/gitlab-org/gitlab-ee/issues/3853#note_47359905
• If the user viewing the page does not have access to view the epic, the link will not be rendered. They will just see plain text.
• You can simply paste the url of an epic in a textbox. Autocomplete is not in scope for this issue.
• Whenever an epic url is saved in a textbox in an object (like an issue or merge request or epic), a system note in the epic itself will be logged, pointing back to that issue or merge request or epic.
• The format is:
  • Victor Wu @victorwu mentioned in issue #3254 2 weeks ago
  • Victor Wu @victorwu mentioned in merge request !3254 2 weeks ago
  • Victor Wu @victorwu mentioned in epic <full-path>/&3254 2 weeks ago
• Note that these system notes will not be visible in epics themselves until https://gitlab.com/gitlab-org/gitlab-ee/issues/3889 is implemented. But they should already be stored in the system.

Edge cases

• None yet

Original description

Support epics references from other gitlab object.

• use & as the prefix
• CE code has to be updated as well (https://gitlab.com/gitlab-org/gitlab-ee/issues/3255#note_41593585 for more details) in order to reserve the prefix (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14854)

Possible issues

• group instead of project is the epic parent
• the code works with different texts on different places - sometimes we use plain version (-> &) sometimes we use html escaped version (-> &) - possible solutions might be
  • to include both & and & as prefix (how to add this to the regex?)
  • investigate if we could always use the same version of the text (probably far too complicated as we transform text on multiple places)
  • change the prefix (however the problem might appear at some point later anyway in case we decide to add new mentionables)

Consider changing skip_project_check to for_personal_snippet to make the filter clearer

/cc @victorwu @smcgivern