Replace the "more ticks / less ticks" bar in number-line with a number-input

Summary: The "more ticks / less ticks" bar in number-line widgets has been causing trouble for students (mostly because it's confusing and awkward), so we're swapping it out for a number-input. Fixes T2915.

Test Plan:
- Add a number line widget
- (make sure "show tick controller" is checked) and play with the tick controller
- make sure you can still submit an answer
- try to break it.

Reviewers: eater, aria

Reviewed By: aria

Subscribers: justin, cameron, marcos

Maniphest Tasks: T2915

Differential Revision: http://phabricator.khanacademy.org/D12146