pidgin/pidgin

e32ab786a04f
Finally add XMPP Message Carbons XEP-0280

Initial XMPP message carbons support. Most of the credit goes to Koosha Khajehmoogahi as I pulled most of this from their GSOC 2015 branch.

Testing Done:
Ran side by side with pidgin2 and messaged a separate XMPP account from pidgin2, pidgin3, and conversations on my phone.

Reviewed at https://reviews.imfreedom.org/r/204/
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