Don't repeat date in each message

[nickmeinhold]

Fixes #172

• code is the result of pair programming on sat meetup
• added extension for comparing DateTimes by date only
• added extension for printing a DateTime as date only
• added abstract MessagesListItem for Message & SectionDate to implement
• added a stream controller to database service so we can return the controller's stream and add to the stream
• change getMessagesStream() to return the stream controller's stream, listen to the database stream, and add section dates
• remove the date from the ChatMessage widget
• change the ChatPage widget to use the MessageListItem type, and check for item runtimeType to display the correct widget
• updated tests so current tests pass (no tests added for this feature yet)

EDIT: added #201 and #202 to track adding tests

[nickmeinhold]

@tourdownunder thanks for a great pair programming session, this was a great team effort and I think we made really good progress, from not being sure how to attack the problem to getting to a working solution... awesome work!