ThemInitialization uses state, should use cache

Problem/Motivation

This was added in #2228093: Modernize theme initialization but:

State is used instead of cache to not rebuild the theme domain object all the time. (as in HEAD)

Doesn't really justify using state.

Proposed resolution

Move this to cache - it could probably be in cache.bootstrap or cache.discovery which would mean we get to use chained fast backend for it.

Will remove a database query per page on warm caches.

Remaining tasks

User interface changes

API changes

Data model changes