Implement XEP 296 for locking resource in conversations.

With a few specific behaviours: When manually opening a conversation with a
bare jid, we open a normal conversation that follows the XEP (locked and
unlocked accordingly). If the user manually opens a conversation with a
fulljid (by selecting a specific resource in the roster, or by specifying a
fulljid to the /message command), we open a special tab that doesn’t follow
the XEP (it is always locked to the same resource, and cannot be unlocked).

When a message is received, unless a special tab has been manually opened by
the other with that specific resource, we always send the messages to a uniq
normal tab, unlocking or locking it according to the XEP.

This means that only one tab can be opened with a given contact, unless the
user specifically chooses to open a special tab for a specific resource.

fixes #2159

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