New conversation flow

[aarmea]

1. On the main screen there should be a new conversation button. When clicked, a new view appears that is dedicated to picking a recipient.
2. The user selects a recipient by typing in part of a username and then selecting from a list of results. When a recipient is selected, a new view appears for the conversation with that recipient. If a conversation already exists, it is loaded; otherwise a new conversation is created.
3. The user inputs text into a text field.
4. The user sends the text.
5. The conversation view is updated to display the sent message.

[aarmea]

I'm thinking of using https://github.com/stfalcon-studio/ChatKit for the actual implementation of the conversation view. The main view when you boot up the app is in noise/views/ConversationList.java, and the identities table is defined in noise/models/RemoteIdentity.java. I already started doing some work to the recipient picker in this commit: 672b35b, roughly following Google's guides on search interfaces and custom suggestions.