Frances/feature/studio nav manipulation2

Submission for <a target="_blank" href="https://edx-wiki.atlassian.net/browse/STUD-1193">STUD-1193 (Read-only view of pages).</a>


1. Added a new CourseTabList and CourseTab class with subclasses for each tab type in common/lib/xmodule/xmodule/tabs.py.
2. LMS and CMS changes to use the new encapsulated classes.
3. Changed Studio to display all tabs presented by LMS in Studio's Pages view.
4. Renamed "Static Pages" to "Pages".
5. UX changes to support Hiding of pages and a new "Add Page" action button.
6. Removed the Waffle "merge_course_tabs" after confirming with @marcotuts, @shnayder, @rlucioni, and @cpennington 

Thank you reviewers: @cahrens @singingwolfboy @sarina @feanil @marcotuts