[Bug web/107297] Support markdown in bugzilla comments

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107297

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> And many folks don't use markdown especially when they are used to plain
> text. C++ is the worst where < gets eaten up. This has been one of issues
> with github even.

Yes, automatically treating plain text as markdown when moving llvm bugs to
github was terrible. Retroactively altering the presentation of old comments is
not acceptable. So let's assume old comments will not be affected in any way
(which is the case for BMO).

But there could be a checkbox (defaulting to off) to interpret the text as
markdown, so the commenter controls it. It would only affect new comments.

Since our bugzilla displays all comments in monospace font, what would be the
benefits of markdown except for displaying preformatted code blocks? Embedded
links seem like spam fuel (currently we only hyperlink URLs and magic strings
like Bug 107297 comment 6 and git revisions). It seems like this is really a
request to support rich formatting, where markdown is just the means to an end,
not the actual goal.

I selected a recent BMO bug to see how it looks:
https://bugzilla.mozilla.org/show_bug.cgi?id=1778863
New comments are indeed in proportional font, with embedded links, quote
blocks, and `code font` spans.