BugWatchActivity isn't recorded if a watch doesn't get checked

[Graham Binns]

What happens:
If a watch doesn't change on the remote server Launchpad doesn't record the fact that the watch was looked at at all in BugWatchActivity, though the BugWatch's lastchecked date gets updated.

What should happen:
A BugWatchActivity entry with a result of something like BugWatchActivityStatus.REMOTE_BUG_UNCHANGED should be recorded and the BugWatch page should show a message for that activity to the effect of "remote bug unchanged."

This is slightly related to bug 586754; it might make sense to fix that bug at the same time.