Backlog: Introduce alt / deadline moving mode

When holding down the shift key, only show
deadlines and stories before the next meeting.

Moving stories in this mode still works, and
is one way to move stories great distances
around the backlog.
Fix: Backlog: Focus summary box on select

We probably broke the auto-focus functionality
when we introduced the ng-if elements to
add the story details to the DOM only when
a story is selected.
Backlog: Rename 'deadline' to 'milepost'

'Deadline' is a bit rough. 'Milestone' was
also considered, but it is a bit corporate.

Eventually these jargon words should be editable
by the client.
Backlog: Special URL for next meeting

Going to /stories/next-meeting will go to
the next meeting.
Backlog: Icon for entering alt / milepost mode