Add an `is_mine_server_name` method, similar to `is_mine_id`.
Ideally we would use this consistently, instead of sometimes comparing
against `hs.hostname` and other times reaching into
`hs.config.server.server_name`.
Also fix a bug in the tests where `hs.hostname` would sometimes differ
from `hs.config.server.server_name`.
Signed-off-by: Sean Quah <seanq@matrix.org>
