imprv: Honor max_reactions_per_user == 1

This commit adds partial support for XEP-0444 v0.2+. In case a contact or a MUC advertises that they only support a single emoji reaction per user per message, gajim will not try to react with more than one emoji. When the user clicks on an emoji, it will treat it as a replacement instead of an addition.

It relies on python-nbxmpp!104 (merged)

Let gajim be the first client to implement this! Changes are minimal and it improves UX with some slidge gateways. Outside of gateways, MUC implementations might implement that too, some day…