EDUCATOR-1001 Fix language for visibility editor and validation to include units

This reverts commit f97f0525, reversing
changes made to 800bcd8e.

TNL-6744

More accurately, this creates a 4th shard because the
last shard is always the default.

TEST_DATA_MIXED_MODULESTORE
Remove these test mixed modulestores:
TEST_DATA_MIXED_TOY_MODULESTORE
TEST_DATA_MIXED_CLOSED_MODULESTORE
TEST_DATA_MIXED_GRADED_MODULESTORE

More accurately, this creates a 4th shard because the
last shard is always the default.

TEST_DATA_MIXED_MODULESTORE
Remove these test mixed modulestores:
TEST_DATA_MIXED_TOY_MODULESTORE
TEST_DATA_MIXED_CLOSED_MODULESTORE
TEST_DATA_MIXED_GRADED_MODULESTORE

There's no need to use a string literal in setattr, delattr, or the
two-argument form of getattr.

Cannot hold on to xblock references.

The existing pattern of using `override_settings(MODULESTORE=...)` prevented
us from having more than one layer of subclassing in modulestore tests.

In a structure like:

    @override_settings(MODULESTORE=store_a)
    class BaseTestCase(ModuleStoreTestCase):
        def setUp(self):
            # use store

    @override_settings(MODULESTORE=store_b)
    class ChildTestCase(BaseTestCase):
        def setUp(self):
            # use store

In this case, the store actions performed in `BaseTestCase` on behalf of
`ChildTestCase` would still use `store_a`, even though the `ChildTestCase`
had specified to use `store_b`. This is because the `override_settings`
decorator would be the innermost wrapper around the `BaseTestCase.setUp` method,
no matter what `ChildTestCase` does.

To remedy this, we move the call to `override_settings` into the
`ModuleStoreTestCase.setUp` method, and use a cleanup to remove the override.
Subclasses can just defined the `MODULESTORE` class attribute to specify which
modulestore to use _for the entire `setUp` chain_.

[PLAT-419]

TNL-652

TNL-651

Co-Authored-By: Christina Roberts <christina@edx.org>
Co-Authored-By: Daniel Friedman <dfriedman@edx.org>
Co-Authored-By: Don Mitchell <dmitchell@edx.org>

TNL-1022

Move lms.lib.xblock into lms.djangoapps.lms_xblock in preparation add database backed configuration to it

TNL-670

TNL-649

[PLAT-191]

STUD-1875

STUD-302, STUD-303

There is an option to:
* Enable/disable accordion navigation
* Enable/disable/repoint tab navigation

This allows for full-screen XBlocks (e.g. a code IDE, or large video
player). It is also the first pass at allowing top-level XBlocks. It's
also now possible to make a chromeless XBlock, point a tab to it, and
make it point back to that tab.

Next steps down that path would be:
* Fix up how tabs are handled. The current version is a hack.
* Create appropriate XBlocks for courseware, tabbed navigation,
  etc. to reach feature parity
* Invert/rejigger the XML format.