CEFR placement test usage report, JSON response bugfix, updated video player, and more

Today's update (2ef4099) brings with it the following changes:

New features

A report showing how many CEFR placement tests have been completed has been added. The report is only available to global administrators and owners of the specified groups. The groups included in the report is specified in the global configuration value placement_test_usage_report_groups.
Added back-end tool take_ownership. With this tool ownership on institutions, groups, courses, containers and objects can be quickly changed recursively.
Added region-agnostic Arabic user-interface language with locale code ar_001 to replace the region-specific code ar_001. The code ar_001 has been demoted to a content language. When rendered in HTML markup, the new code ar_001 will show up as a lang="ar" attribute. As this locale has no region associated with it, a white flag is used where required.

Updated video player to mediaelement.js 2.18.1 for better compatibility with newer browsers.
Extended database to handle locale codes longer than 5 characters. This was required by the Arabic change mentioned above.

Fixed issue where JSON response text sometimes would return garbled Unicode text (Mojibake) and the wrong content length. Returning the wrong Content-Length HTTP header would cause all sorts of hard-to-debug issues, usually related to keep-alive connections and invalid byte boundaries. All use-cases where encode_json() was used have been reviewed and fixed.
When a user was removed from an institution, they might not have their student/supervisor relationship revoked. This bug should now be fixed.

The user-interface language ar_001 has been demoted into a content language. You should use the locale code ar_001 instead for setting UI language.