As a commenter I want JIRA issues mentioned in comments to be hot-linked

As a commenter,
I want JIRA issues mentioned in pull-request comments to be hot-linked,
So that when I am discussing a particular pull-request, if I mention a ticket it can be easily observed by other members of the team viewing the discussing.

Scenarios:

• "Yeah, that looks like a bug, I've raised TICKET-100 to fix this going forward, but for now we can go ahead and merge"
• "I'm not happy with this change, TICKET-120 will be much harder to complete"

Possible progression for delivery;

1. Initial lightweight integration where Stash simply recognises issue keys and links to the JIRA instance associated with the project/repo
2. Slightly tighter integration using a similar UI to the issue view on the pull request (ie pop-in window frame) to show a summary without hopping to JIRA
3. Bidirectional integration where JIRA gets a trackback reference, and the stash ui includes the mentions as if they were mentions in commit messages