Our implementation requires at least 1 space after the
'#' not not break existing linkifiers like '#123', etc.
that generally follow the convention we show in linkifier
examples.
- [valid] : # Hello
- [valid] : # Hello
- [invalid]: #Hello
For the frontend, we have taken the code from v0.7.0 of
upstream marked and made minor changes to avoid having
to refactor a significant part of our marked code.
For the backend, we merely have to change the regex to
force require spaces after #, and add hashheader to our
list of blockparsers.
Fixes#11418.
This moves our main CSS for rendered Zulip message content into an
external file, which may be reusable but in any case should make it
easier to find this content.