Clean up spacing of Schedule Filters

[jonseitz]

The design and spacing of the header for the schedule needs to be cleaned up, as in the prototype.

@Vittorio2015 to update prototype to make schedule dropdown more prominent).

[jonseitz]

Testing

Changes should only be in CSS, so tests should still pass as expected. Potentially could be issues if tests are traversing/navigating DOM trees.

[Vittorio2015]

Prominent academic year dropdown

• Moved Undergraduate and Graduate checkboxes to the left and within a dropdown
• Moved Cambridge and Allston checkboxes within a dropdown
• Center aligned academic year label and dropdown
• Increased font size

Source: https://seas-computing.github.io/course-planner_ux/schedule.html also at https://github.com/seas-computing/course-planner_ux/tree/gh-pages

Example of selected buttons

Source: https://seas-computing.github.io/course-planner_ux/schedule.html also at https://github.com/seas-computing/course-planner_ux/tree/gh-pages

[jonseitz]

Closed by #588