Add support for KaTex

[KarolTrzeszczkowski]

• I have searched open and closed issues for duplicates

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Feature request description

It would be so great if you added KaTeX support. Facebook (desktop version) has it and it's very useful. I know you have a lot of stuff to do but for many of my friends it would be the main argument to join Signal.
https://github.com/Khan/KaTeX

This is in javascript so it might be possible to do it

Steps to reproduce

write a latex formula in $$ surrounding and send it to another person

Actual result:
you get just a bunch of $$\frac{e^x}{etcetera}...$$ signs
Expected result:
You get a pretty mathematical formula for the people of culture.

Screenshots

How it looks like in facebook

[scottnonnenberg-signal]

Yes, it is in javascript. But are there libraries to support it for iOS and Android apps? We'd want to be sure that if Signal Desktop could send/receive it, our other Signal apps could send/receive it.

[KarolTrzeszczkowski]

For Android I've found this https://github.com/lingarajsankaravelu/Katex
and this https://github.com/ianarawjo/KaTeX-iOS for iOS.
I'm no developer so I can't really settle how useful it is, but maybe it is :)