Feature request: Preserve MathML (Core) tags

(Related to #327920 (closed).)

GitLab strips tags which aren't allowlisted here from the rendered HTML.

Since MathML (Core) is now available for 90+% of web users and growing (available in all relevant browsers, https://caniuse.com/?search=mathml), I suggest to allow those tags in READMEs etc. This circumvents various problems existing in the current math implementation, e.g., the limitation to 50 rendered math fields per page (a restriction introduced to speed up page loading, no longer relevant with MathML).