Add more full-featured email functions

When you're working with a team, sometimes short chats won't do and you need to send long messages. The chat right now requires you to do shift+enter to add lines in your message, which does not come intuitively to my non-techy coworkers who I'm trying to convince to abandon email for Keybase. Also, there is no larger window that allows you to more easily compose long messages, nor is there any text editing functionality. If these features could be added to chat it would be much easier to abandon email altogether.

The issue does not name any files, tests, or entry points. Start by locating the chat composer in the client and review how multiline input, composing windows, and text editing are currently handled; done means long messages can be composed with intuitive line breaks, a larger editor, and text-editing functionality.

Written by the indexing model from the issue text.