Fix matrix api joinRoom() for remote rooms (#60)

When using self-service mode and trying to link with a remote matrix
room (room not in the same HS as the bridge user), then we need to add
the "via" HSs to join the room with, or else it fails.

We get it from the "m.space.child" in the "children_state" of the space
hierarchy.

Co-authored-by: Cadence Ember <cadence@disroot.org>
Reviewed-on: https://gitdab.com/cadence/out-of-your-element/pulls/60
Co-authored-by: Elliu <elliu@hashi.re>
Co-committed-by: Elliu <elliu@hashi.re>

test/test.js

@@ -17,6 +17,8 @@ const {reg} = require("../src/matrix/read-registration")
 reg.ooye.discord_token = "Njg0MjgwMTkyNTUzODQ0NzQ3.Xl3zlw.baby"
 reg.ooye.server_origin = "https://matrix.cadence.moe" // so that tests will pass even when hard-coded
 reg.ooye.server_name = "cadence.moe"
+reg.ooye.namespace_prefix = "_ooye_"
+reg.sender_localpart = "_ooye_bot"
 reg.id = "baby"
 reg.as_token = "don't actually take authenticated actions on the server"
 reg.hs_token = "don't actually take authenticated actions on the server"