Don't require a device be registered in Almanac to do cluster init/resync steps

Summary:
Fixes T12893. See also PHI15. This is complicated but:

• In the documentation, we say "register your web devices with Almanac". We do this ourselves on secure and in the production Phacility cluster.
• We don't actually require you to do this, don't detect that you didn't, and there's no actual reason you need to.
• If you don't register your "web" devices, the only bad thing that really happens is that creating repositories skips version initialization, creating the bug in T12893. This process does not actually require the devices be registered, but the code currently just kind of fails silently if they aren't.

Instead, just move forward on these init/resync phases even if the device isn't registered. These steps are safe to run from unregistered hosts since they just wipe the whole table and don't affect specific devices.

If this sticks, I'll probably update the docs to not tell you to register web devices, or at least add "Optionally, ...". I don't think there's any future reason we'd need them to be registered.

Test Plan:
This is a bit tough to test without multiple hosts, but I added this piece of code to AlmanacKeys so we'd pretend to be a nameless "web" device when creating a repository:

if ($_REQUEST['__path__'] == '/diffusion/edit/form/default/') {
  return null;
}

Then I created some Git repositories. Before the patch, they came up with - versions (no version information). After the patch, they came up with 0 versions (correctly initialized).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12893

Differential Revision: https://secure.phabricator.com/D18273