* CSS & HTML tweaks pending UX review

* Update translations

Add info about Courses page filtering and Course Summary report download

* Raise 404, not 500, if invalid course_id requested

* Remove unneeded try/except in CourseContextMixin

The Http404 will be raised by the middleware or by the CourseValidMixin before
it even starts executing that function.

* Revert adding try/except to format_course_key tag

* Move summary metrics to separate function

* Add summary metrics to index w/ acceptance tests

* Update translations

* Fix linting errors

* CSS tweaks and adjustments

* Fix course_index acceptance tests

* Address PR comments and refactor acceptance tests

* Less awkward cards header wording

* Update translations

Update translations

* Use new remove_keys util to remove cols from CSV

Removes created and enrollment_mode.*.count_change_7_days columns from the
course-list CSV. Includes unit test for remove_keys util function.

* Fix CSV test

* Add download csv button to course list page

Also, a11y fix: make visible results-num span aria-hidden so the text is not
read twice by a screen-reader.

* Add acceptance test for downloading csv

* Use non-deprecated karma gulp runner

* Update jasmine-core npm package

* Output node version before running tests

* Try updating all karma-related npm packages

* Update translations

* Add disabled/active sr text to pagination links

* Update translations

Pin backgrid to 0.3.7 (0.3.8 introduces a breaking change in Insights).

Update translations

* Course list CSV endpoint with unit tests

* Replace custom csv rendering with drf-csv function

* Refactor CSVResponseMixin to be DRYer

Change edx-django-release-util version.

Add course listing filters.

We no longer use the "release" branch.

* Add header tooltips to the course-list table

* Update translations

* Fix eslint error

Update translations

Upgrade nodeenv to 1.1.1

Fixed course list search event naming.