Add messages to correct channel when joined from two accounts/spaces

[raucao]

When you join a channel from a second account, both incoming messages (of every message) get added to the first occurence of that channel name, irrespective of account. (We never needed this with IRC, but with XMPP we can join the same channel from multiple accounts/spaces).

[raucao]

After thinking about the UX of joining a channel from two different accounts, I have come to the conclusion that we probably shouldn't make this possible in the first place. There is no real benefit to this in actual use, but quite a few drawbacks when it comes to usability and UI design. So I'll close this for now, but please feel free to re-open in case it's important to you.