Add support for database flavors to `swh db-init`
This adds a "flavor initialization step" to the initialization of the database, after the execution of the sql file named *-flavor.sql.
I've added a few sanity checks to warn if one of the underlying assumptions for this feature breaks.
Test Plan
called swh db-init with a bunch of combinations of valid and invalid flavors to check the behavior.
Migrated from D3997 (view on Phabricator)