The configuration template only rendered a datastore when its for_kinds
list was non-empty, even though the inline documentation (matching
upstream's) describes forKinds: [] as the way to make a datastore
readonly. That made the documented migration scenario impossible: moving
media between the file and s3 datastores while keeping the old one
readable.

Introduce matrix_media_repo_datastore_file_enabled and
matrix_media_repo_datastore_s3_enabled, which default to the previous
kinds-based behavior. A readonly datastore is now expressed by enabling
the datastore explicitly while assigning it no kinds.

Fixes #4303

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>