Places frontend initialization is surprisingly complex, and disentangling it
from the rest of startup by moving it to its own file helps make some of the
logic a little more obvious, and makes unit-testing a bit easier.
This also removes BrowserGlue from the indirection mechanism used between
MigatorBase instances and Places, by switching to category-manager-based
invocation - this way, migrator code does not need to directly
call places code to tell it it's done, but we don't need BrowserGlue to play
messenger between them.
It would be nice to do the same thing for `places-init-complete` but
that is notified from C++ code so unfortunately that is not easily possible.
Differential Revision: https://phabricator.services.mozilla.com/D244428
78ebe33ed8