Sending message to room with a slash in the room ID fails

[#test:maunium.net](https://matrix.to/#/#test:maunium.net)'s room ID is `!test/v6:maunium.net`. When trying to send a message to the room, Element iOS fails with "Messages not sent". Other clients like Element Web and Android work fine.

The problem is most likely that someone forgot to URL-encode the room ID (it's an opaque string, so you can't assume that it's url-safe even if they usually are)


Reproduce the failure in Element iOS by sending a message to the room ID `!test/v6:maunium.net`, then trace how that room ID is placed into the send-message request. Done means messages send successfully for room IDs containing `/`, while existing room messaging continues to work.

Written by the indexing model from the issue text.