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.
* DEV: Use Ember CLI middleware to decorate the index template
Previously we'd do this on the client side which did not support our
full plugin API. Now requests for the index template will contact the
dev server for a bootstrap.json and apply it to the current template.
* FIX: Allows logins in development mode for Ember CLI
Discourse needs a bunch of data preloaded before it can start up.
Normally we throw blobs of this into the HTML document that is requested
but in some cases that's awkward to retrieve.
For example with Ember CLI you have a separate javascript application
that needs to make its own HTML.
This API endpoint returns a JSON object with all the data Discourse needs to
bootstrap and start up.
