Add links to change pages on /zuul/status page.

The links to Jenkins job pages on the /zuul/status page are awesome,
but now we need links to go the other direction and link back to
Gerrit. Add links using the event['change']['url'] parameter as the
target.

Change-Id: I24f189fd0d14e59e0ae5484b15ea6e3aa899e62f
2 files changed