Reopen jira issues on Reopens keyword and add sha to issue for detected reverts

A new keyword, "Reopens:", is proposed for commit message footers which will trigger the reopening of a Jira issue.

Proposed behavior is twofold:

1. When the Reopens footer appears in conjunction with the Fixes footer, it will reopen the specified issue upon pushing the first patchset to Gerrit. (F.ex. A follow-up change to fix an issue by which the first patch that closed the change did not completely resolve the bug)
2. When the Reopens footer appears without the presence of Fixes, it will reopen the specified issue upon merging the change. (F.ex. A revert that once merged makes the tagged Jira issue outstanding once more)

Jira issues implicated by Reopens should be tagged with the relevant commit once merged.

Further, changes which include a revert in the commit message should tag the relevant Jira issue with the merged sha, even if the Reopens keyword or any other keyword is not present on the change. This relation can usually be identified by examining the original commit the revert relates to.