Allow admins to 'force-queue' a postponed test

Currently, HEAD failures cause a bunch of tests to become postponed ... even if it is a false positive.

When it's a false positive, it would be nice to be able to:

i) Force the HEAD test back to a 'passed' status,
ii) Force an individual test back to 'queued' status,
iii) Force all postponed tests for a branch back to 'queued' status.

When it is a legitimate HEAD failure, ii) above would allow us to test the patch which is supposed to fix the break before it is committed ... assuming we're actually patient enough to wait for it to run. ;)