Cleaned formatting/comments in PasswordResetFormTest
Fixed #21291 -- Ensured inactive users cannot reset their passwords

Thanks kz26 for the report and the suggested fix. Refs #19758.
Removed unused local variables in tests.
Fixed #21283 -- Added support for migrations if models is a package.

Thanks Markus Holtermann for the report.
Added support for serializing class methods. - Refs #21290.

The new handling allows us to do away with the whitelisting that was
required to support date and datetime objects.
Add thread-safe CacheHandler
Add DefaultCacheProxy for old 'cache' object
Remove thread locals use in memcached backend
Update docs
Simplify docs
Add notes to release doc