Implement Pump.io APIs

At the moment this supports:
* OAuth authentication
* main activity, favorites, outbox and inbox timelines
* post sending; sent to Public by default
* private post sending; a post sent with custom To/CC fields
* medium upload (I tried with images, but should work with any supported type)
* images previews
* like/unlike any post
* reshare any post
* deletion of a post the user wrote
* show post replies in a separate widget

BUG:319348