…lugins for matrix things (#727)

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### Description

<!-- Please include a summary of the change. Please also include
relevant motivation and context. List any dependencies that are required
for this change. -->
Replaced Cinny's custom markdown parsing with marked which should
drastically reduce the various random obscure errors and crashes caused
by it.
Added math rendering support with KaTeX, sent using standard formatting
of $ for inline and $$ for block with LaTeX.

Supersedes #726
Fixes #711 
Fixes #587
Fixes #467  
Fixes #684 

Tbh I doubt I've tested this enough so there's probably still a bunch of
bugs, especially in parsing html back to markdown, but hopefully it's
less than our current system?

Might consider using another library like Turndown for converting html
into markdown, currently using a partial custom parser with... well okay
results.

Updated to also remove the WYSIWYG editor because it's largely unused,
can be entirely covered by markdown, and complicates translating html to
markdown. Keyboard shortcuts from the editor have been rewritten to
instead apply markdown syntax.

#### Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [x] Breaking change - removes the WYSIWYG editor.
- [ ] This change requires a documentation update

### Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings

### AI disclosure:

- [x] Partially AI assisted (clarify which code was AI assisted and
briefly explain what it does).
- [ ] Fully AI generated (explain what all the generated code does in
moderate detail).
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explanation using AI!! You must prove you understand what the code in
this PR does. -->

Tests were AI generated and then reviewed by me. I can confirm that the
tests probably test the right stuff.
I had AI do the initial merge and write the marked plugins for the link
preview hiding and spoiler hiding features, which I then tested and
reviewed.