Prevent public channels with identical names from being created

[nesium]

Public Channel names should be unique, i.e. it would be weird to have two #general channels in the same company.

Before creating a public channel, the core-lib currently loads all MUC rooms, checks if a room with the given name already exists and throws an error in that case. The same is true when our user tries to rename a public channel. This logic is prone to race conditions, i.e. client 1 and client 2 check for duplicates with no duplicate being found then create a room with the same name. The logic is also not in effect when using another XMPP client, so uniqueness could only be enforced server-side.