Restructure exercise to be more efficient

Summary:
Get ParallelLines to work with rotation

Get vertical angles exercises working with new ParallelLine function

Improve hints

Restructure parallel_lines 2 to allow for easier editing

Allow labels to be removed and update question wording

Fix data-ensure issue

Add degrees to hint equations

Standardize question formats and wording

Add degrees symbol to hints in vertical angles

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D9822