Remove dependency on vue-showdown

Depend on Showdown directly instead, cutting out the middleman.

Additionally:

* Escape HTML outside of Markdown code blocks, in order to prevent unintended rendering of HTML elements (e.g. the `textarea` in CVE-2022-25869)
* Always run Showdown output through DOMPurify

Signed-off-by: nscuro <[email protected]>

Author: nscuro