Story detail view cleanup

This commit collapses the multi-column view into a much more compact
vertical layout, and with the added screen realestate adds more
obvious edit controls for tasks.

- Collapsed story detail view into a single column.
- Added better edit toggle controls for the story and task edit forms.
- Pulled task edit form out into its own custom directive so we can
use it in multiple places.
- Added disabled controls for author editing, since filtering
thousands of users needs to be implemented on the backend first,
and the assignee_id patch hasn't landed yet.

Change-Id: I92b05ca7ef4b2bb4d0cc536eb2acf278249e2fe6