Do some WebHook-related cleanups

Review Request #8223 — Created June 7, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
f3bd51b...

Reviewers

The default fieldset for WebHooks in the admin form now has a label
instead of None. In addition, the __str__ method of
WebHookTargets now just returns the URL, which is a more readable
string for viewing in the admin panel.