When an application updates metrics for a toggle we now
stores the timestamp on the toggle when it was last seen
used by an application. This will make it much easier to
detect toggles not in use anymore.
closes#642
Will know tell the user if the toggle name is already
in use by an active feature toggle or an archived toggle.
Also brings lates unleash-frontend fix, which prevents an
invalid form from submitting.
closes: #290, #291
