Testing migration from old mongo to split.

Tests verify operation counts for old mongo. Should be extended to split in the future.

Cannot create an xblock in a course that doesn't exist.

make_asset_key does not start with "/" for split locators.

Note that now 2 courses can be imported where the only difference between them is run.

export_courses_to_output_path uses old mongo locator to create directory structure.

The way that assets are accessed (the usage of _id) seems specific to old mongo and fails with split.

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-836

These were found by the new pylint checker I wrote! :)

This provides a better UX for video uploaders, because recent uploads
are readily visible at the beginning of the table.

TNL-1185

Context: We have witnessed multiple, seemingly random "CSRF verification
failed" errors while signing in (with valid ID) to the Studio.

Explanation: The login form does not initially include a CSRF field.
The CSRF header of the request is appended to the studio login request
headers by intercepting the form validation. This intercept is performed
by the login.js script. Unfortunately, the login.js script is loaded
pretty late (at the end of the template). So if the login form is
validated sufficiently fast, the login.js script has no time to load and
append the X-CSRFToken header to the request.

Proposed solution: the CSRF token is already passed to the template via
the login view, so we just add a hidden field to the login form to
include the csrf token.

This fixes two bugs:
1. All statuses were listed as "Unknown"
2. Non-ASCII characters in a header column translation resulted in a 500

TNL-935

This fixes two bugs:
1. All statuses were listed as "Unknown"
2. Non-ASCII characters in a header column translation resulted in a 500

Fix i18n for video status strings (broken in commit 4b53f4df) and remove
unnecessary complexity from a test case. This also removes the status
whitelist in the video upload configuration. Because status is included
in the CSV report, it is not necessary to filter the included videos by
status.

returns XBlockuser with 2 data layers

TNL-935

Possibly caused by a bad merge.