compose: Quote and reply

[chrisbobbe]

[chrisbobbe]

It might be helpful to add a quote_and_reply.dart file somewhere. Generating the quote-and-reply text has some nontrivial subtasks:

• Silent @ mention of the sender
• Link to the message (different for PM and stream messages)
• Fencing with ```, but with the right number of `'s depending on the message content
• (Leading and trailing newlines, if they're considered part of the text.)

And a new file with that name could be a good home for all that code. Where might be a good place to put that file? Maybe not "model"…or "widgets"…etc.

[chrisbobbe]

Greg points out in the office just now, model would be a fine place for it.

[chrisbobbe]

#140 will be helpful as a way to get the message's raw Markdown content.

[chrisbobbe]

With #139 being merged, we can consult store.streams when writing a stream-message link. (Then when we update store.streams on stream events, tracked in #135, those links will naturally get written with up-to-date data.)