Previously we were compiling core and theme CSS into two targets:
Desktop and Mobile. The majority of both files was the 'common' css.
This commit splits those common styles into their own targets so that
there is less duplication. This should improve compilation times + cache
reuse, as well as opening the door for experiments with
media-query-based mobile-modes.
The only functional change is that we can no longer use `@extend` to
copy 'common' rules in core to mobile/desktop. This is probably for the
best. Duplication and/or mixins are a more native-css pattern for this.
Plugins already have a common / mobile / desktop pattern, so are
unchanged by this commit.
Before this change, calling `StyleSheet::Manager.stylesheet_details`
for the first time resulted in multiple queries to the database. This is
because the code was modelled in a way where each `Theme` was loaded
from the database one at a time.
This PR restructures the code such that it allows us to load all the
theme records in a single query. It also allows us to eager load the
required associations upfront. In order to achieve this, I removed the
support of loading multiple themes per request. It was initially added
to support user selectable theme components but the feature was never
completed and abandoned because it wasn't a feature that we thought was
worth building.
