Adds an endpoint which will always result in a clobber when accessed by ...

...the client

Some builders are configured in such a way as to always require clobbering,
this is currently acheived by pointing them at the endpoint provided by:
https://github.com/mozilla/build-tools/blob/master/clobberer/always_clobber.php
Which simply returns a lastclobber time of the current time + one hour.
The forceclobber endpoint re-implements this behavior.