Render references in content editor

What does this MR do?

Issue: #338531 (closed)

GFM supports syntax for referencing issues (#2525235), milestones (%13.4), labels (~UX), MRs (!33636), etc. This MR adds support for them in the content editor.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Edit a Wiki page.

2. Type the below code:

   Hi @gitlab - thank you for reporting this ~bug (#1) we hope to fix it in %1.1 as part of !1.

   Note that the references above will convert to links only if the references actually exist. So please create some users, labels, issues / MRs before referencing them.

3. Click "Use new editor".

4. The content editor will automatically convert the elements to reference links as you see them live.

5. Click "save". The content should be serialized back to the original reference.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #338531 (closed)