UX changes for course navigation manipulation
Changes for viewing built-in tabs in studio

Changed "Status Page" -> "Page".

UX:
    support for displaying built-in tabs
    restored drag and drop on Studio Pages
    additional styling for fixed state on Studio Pages
    add a new page action added to bottom of Studio Pages

Dev
    changes for viewing tabs in studio,
    refactored the tab code,
    decoupled the code from django layer.
    is_hideable flag on tabs
    get_discussion method is needed to continue to support
external_discussion links for now since used by 6.00x course.
    override the __eq__ operator to support comparing with
dict-type tabs.

Test
    moved test code to common,
    added acceptance test for built-in pages
    added additional unit tests for tabs.
    changed test_split_modulestore test to support serializing objects
that are fields in a Course.

Env:
    updated environment configuration settings so they are
    consistent for both cms and lms.
changes per PR comments.
more code coverage.
changes to address PR review comments.
  i8n - more Translators notes
  to_json - call superclass methods
  assert_true vs assert_equal
  pylint comment
  move inherit base file to top of html file
updated CHANGELOG
1. quality fix in pages.py
2. renamed CourseTab.from_dict to CourseTab.from_json to be consistent
   with other such APIs in the xmodule library. Updated comment to
   specify that the expected input is a json-like structure.
3. changed implementation of key_checker per DB's suggestion.