This change should make it more obvious which order the steps must be done

Since we'll likely have generic SQLite database importing
via [pgloader](https://pgloader.io/) for migrating bridge
databases from SQLite to Postgres, we'd rather avoid
calling the "import Synapse SQLite database" command
as just `--tags=import-sqlite-db`.

Similarly, for the media store, we'd like to mention that it's
related to Synapse as well.

We'd like to be more explicit, so as to be less confusing,
especially in light of other homeserver implementations
coming in the future.

This flag is necessary for Ansible to ask for the sudo password when using the non-root option as documented in the `hosts` file. Otherwise, Ansible errors out with `missing sudo password`.

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.