Add support for Markdown and KaTeX

[Bartozzz]

Users should be able to use Markdown and KaTeX in their posts, both in threads and comments.

[unrooted]

Okay, here are our options for how to do that:

1. We can use modules like Vue-Katex and Vue Markdown. They're npm packages, and they can help a lot.

2. Recently, you've said something about using Markdown as a special component in our code. Could you please, explain exactly what to do?

About 1st option, I don't exactly know how to do it, so the research can take a while, actual work time could be less than research time.

And, I dunno what to say about 2 option. But using Markdown elements as a component could take less time, less research than option 1.

[Bartozzz]

I'll take care of this feature. The best solution will be to create a wrapper (Vue component) over a markdown parser.