Allow well-formed links in Markdown
While we fixed #1686 (closed), we also disabled a ton of other nice URIs. We should find a way of enabling "all" good links in a way that's not an allowlist, because we want to allow all sorts of new URIs from the future, and the allowlist could potentially grow insanely large.
Examples of links we should ACCEPT:
Examples of links we should REJECT (i.e. render as plaintext):
- Anything beginning with
/(which would be interpreted as filesystem)
- Anything that would crash Manyverse or bug it somehow