If a Postgres dump contains ALTER TABLE ... OWNER_TO <username>
statements which set the owner to a username different from
'synapse' the post Postgres import task will fail complaining
about lack of role.

Changing the matrix_postgres_connection_username group var has no
effect. However, the ALTER TABLE statements (and accompanying comments)
can be rewritten to change the username to 'synapse', which permits the
import task to succeed.

From a sample of 1, having the owner set in this was causes no
discernable side effects on the homeserver.

This is a simplification and a way to make it consistent with
how we do Postgres imports (see 6d89319822), using
files coming from the server, not from the local machine.

By encouraging people NOT to use local files,
we potentially avoid problems such as #34 (Github issue),
where people would download `media_store` to their Mac's filesystem
and case-sensitivity issues will actually corrupt it.

By not encouraging local files usage, it's less likely that
people would copy (huge) directories to their local machine like that.