fix(RichText): always load Mention extension in rich text editor

[mejo-]

Brings back mention bubbles in MarkdownContentEditor.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.53%. Comparing base (b0a250c) to head (0415850).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7419      +/-   ##
==========================================
+ Coverage   52.62%   59.53%   +6.90%     
==========================================
  Files         494      493       -1     
  Lines       42291    37390    -4901     
  Branches     1088     1087       -1     
==========================================
+ Hits        22257    22261       +4     
+ Misses      19927    15023    -4904     
+ Partials      107      106       -1     

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