Secret conversations

[kisamoto]

Users should have the option to have a conversation in "secret" mode, that doesn't save a message to a conversation.

Similar to "private browsing" in a browser that doesn't store any history/cookies.

When enabled, we should make it clear to the user with:

• color change/highlight
• close the sidebar (if open)
• Update the text to say that the messages will disappear when they move away from the chat.