Privacy consent requirements were being registered during Review Board's

initialization phase, and then rebuilt and re-registered whenever

settings changed. The problem was that registering

PolicyConsentRequirement during Review Board initialization would fail

if the database wasn't yet set up. This core problem was addressed in

another change, but still impacted other management commands, and is

really a practice we should aim to avoid.
Djblets now has a new signal for notifying when a registry is

populating, which we're now able to use instead. This means that instead

of registering on startup, we just register when the registry needs us

to, saving startup time and ensuring we don't interfere with any

database setup process.
This also has the benefit of simplifying the logic considerably. We no

longer need to track whether we've previously registered anything, or

deal with forcefully registering consent requirements. We just

unregister old ones and register new ones.

Unit tests pass.
Temporarily reverted an older change for fixing syncdb with initialization,

and verified that the consent registrations no longer trigger breakages with

new databases.
Tested viewing and changing consent for policy and gravatars, then changing

settings that triggered new consent requirement registrations, and then viewing

the consent again. Saw the new information. Repeated this test several times.