Issue page should list commits referring to it (BB-4864)

Commit messages that include an issue number automatically have that issue number turned into a link to the issue.

However, the issue pages themselves have no indication that a commit is referring to them - ie. there's no traversal in the other direction, from issue to referring commit.

It would be ideal if there were a "Referring Commits" section added to the issue page, linking to the commits referring to that particular issue.

As it stands now, I manually paste the truncated commit hash into a comment to create a link from an issue to a related commit, but it would be nice if this were not necessary. It would be great to have a bi-directional linking relationship established solely from referencing the issue number in the commit message.