New activity for sharing, destruction of RecipientsPanel

ShareActivity is essentially a simplified version of ConversationListActivity that is supposed to be a more succinct list of open conversations sorted by recency. The "+" in the ActionBar will let the user create a new conversation to share to instead.

I tried making it a dialog since I preferred that look, but with the way we're currently processing through RoutingActivity, it appeared not-so-possible after excessive and fruitless tinkering with android's style/theming.