Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.

AppStaticStorage only provided one thing over FileSystemStorage, which was
taking an app name (import path) and translating it into a filesystem
path. This is now something that should be done via app_config.path instead,
leaving AppStaticStorage with no reason for existence. It should be safe to
remove, as it was undocumented internal API.

There was some kind of feature in the AppDirectoriesFinder code related to a
"prefix" attribute on the storage class used by AppDirectoriesFinder. Since
this feature was undocumented, untested, and of unclear purpose, I removed it
as well.
Fixed #21829 -- Added default AppConfigs.

Thanks Russell for the report, Marc for the initial patch, Carl for the
final review, and everyone who contributed to the design discussion.
Merge pull request #2211 from carljm/t21867

Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.
Fixed a failing schema assertion.

BooleanFields are stored as TINYINT(1) on MySQL.
Merge branch 'master' into t17304

* master:
  Fixed a failing schema assertion.
  Fixed #21829 -- Added default AppConfigs.
  Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.