To go along with issue2551180, it would be nice to allow:

  issue2551180#msg7413

  issue 2551180#msg7413

be generate hyperlinks to https://issues.roundup-tracker.org/issue2551180#msg7413

Currently I think it only generates https://issues.roundup-tracker.org/issue2551180
or possibly two links one to the issue and one to the message.

This may be tricky to make work for the markdown tests.
They call _hyper_repl_item in templating.py and their patterns
may need to be changed to support this.

I suspect the regexp needs to look like:

  (?P<item>(?P<class>[A-Za-z_]+)(\s*)(?P<id>\d+)(#(?P<frag>[^][#%^{}\"<>\s]+)?)

so add an optional #fragment not including the characters shown plus space characters
as we use those as the end of fragment delimiter. This will not handle %20 url
encoding to allow spaces but for a first pass this should work.