UI work: try to make Flow look vaguely like Brandon's prototype.

Includes the start of some JS, including post and reply with AJAX.

Also includes a refactor of the post display code (post rendering
now has its own template) and an update of the error messages
(actual messages are now provided).

Change-Id: I4bbc64a9fb12a142a882691ac9a59674e0be9ae0
Merge "UI work: try to make Flow look vaguely like Brandon's prototype."