ENH: Linkify links to issues and PRs

[westurner]

Links to issues and PRs are not links in mattermost.
For example, these are not linked:

- A link to #3
- A link to repo#3
- A link to orgname/repo#3

One way I can think of to fix this would be to convert the links to markdown before POSTing them to mattermost. IDK where that function is?