Remove phase from motion and fix close date issue

When an admin closes a proposal the close date displayed is the date it was supposed to close not the date is was closed.

This area of the code is the oldest in the codebase and very messy.  This refactor separates out the functionality of 'tracking when the motion is closing' and 'if it has closed' into two different fields so there is no interference.
Scopes and queries become a lot cleaner as a result.

- Create a new column closing_at populated with close_at data
- Rename close_at to closed_at and set this field to NULL for any motions in phase 'voting'
- Remove column 'phase'.  (an 'open' motion is now, any motion with 'closed_at IS NULL')
- Change validate presence to closing_at
- Update all instances of close_at to closing_at or closed_at
- Create methods open? and closed? for motion
- Create open_motions and closed_motions methods for user, group and discussion
- Simplified 'close_motion' to close!
- Add a step to features/motions/close_proposal to make sure closed_at matches