Improve rendering of KaTex/Math text

[Ezio-Sarthak]

After the merge of #1024, ZT now renders the math/katex/latex text as raw source. Although this retains the markdown information, this is ideally not the rendering a normal user would expect.

Some ideas of implementation:

• A raw -> urwid approach might be used, wherein we convert raw source (current output) to urwid-displayable content. We currently support rendered -> urwid, and thus this might need some investigation.
• One central aspect might be to use a Zulip-specific markdown. We can query the server if this is something that would be beneficial to have such a standardization. Once that's done, we could just grab and render the response.

Previous discussion: #zulip-terminal > Multiple Katex markups display issue