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>

src/d2m/converters/message-to-event.js

@@ -207,6 +207,7 @@ async function attachmentToEvent(mentions, attachment) {
  * - alwaysReturnFormattedBody: false - formatted_body will be skipped if it is the same as body because the message is plaintext. if you want the formatted_body to be returned anyway, for example to merge it with another message, then set this to true.
  * - scanTextForMentions: true - needs to be set to false when converting forwarded messages etc which may be from a different channel that can't be scanned.
  * @param {{api: import("../../matrix/api"), snow?: import("snowtransfer").SnowTransfer}} di simple-as-nails dependency injection for the matrix API
+ * @returns {Promise<{$type: string, $sender?: string, [x: string]: any}[]>}
  */
 async function messageToEvent(message, guild, options = {}, di) {
 	const events = []