Create "special" channel when community is created

[0x-r4bbit]

This is a sub task of the Community History Archives MVP.

Draft spec: status-im/specs#164
Roadmap / Tasks: https://hackmd.io/@status-desktop/B1naRjxh_/%2FNeX8-q49TtqN18i4VkXRFw

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We need to introduce a new "special" channel for communities, which is used to distribute magnet links. The channel is "special" in the sense that:

1. Only comunity owners should be allowed to send messages to that channel (technically, this is not enforced on the transportation layer, so apps (mobile, desktop) should take measures). Messages that weren't sent by the owner in that channel should be ignored by other clients
2. The channel should exist for every community, maybe need to implement a routine that ensures this is true for already existing communities
3. The channel shouldn't be visible in UIs so we might need a new chatType to allow clients to filter it out

[0x-r4bbit]

Superseded by #2565