Fixed #21242 -- Allowed more IANA schemes in URLValidator

Thanks Sascha Peilicke for the report and initial patch, and
Tim Graham for the review.
Added generation of SQLite FK DDL in initial migrations.

SQLite accepts the relevant standard SQL (although by default it doesn't
enforce the constraint), and the 'traditional' creation backend helper
generate it, so this allows us to:

- Maintain the status quo
- Improve readability of the SQL code generated for that backend.

Also, we will need this for when we fix Refs #14204.
Refactored the migration signals to use app configs.

De-aliased pre/post_syncdb to pre/post_migrate to increase
backwards-compatibility.