Add participants preview in popover to conversations

Hey Guys,

Navigating through the repository issues I saw two feature proposals (#3965 and #3873) that I found useful, interesting, and not too hard to implement. I gave it a shot, following a new idea that came to my mind. Its just a new link on each conversations that displays at most 20 users, and their respective hovercards, to see who is involved on each conversation without loading each one. Its my first feature on diaspora, hope you like it, feel free to make suggestions. Besides that I made some refactors of things I wasn't so happy with.

Cheers!