Ensure all user-facing cohort-related strings are internationalized

TNL-55

I've done a run-through of all our various cohort UIs to verify that we are in fact translating user-facing strings.

The only problem I found was in the cohort editor, and seems to be caused by a bug in the translation scraper which doesn't handle newlines too well.
@explorerleslie There's also a question about the student profile CSV - see below.

Places I've checked (did I miss anything?):
* Studio
  * Advanced Settings - translated
* LMS
  * Instructor Dashboard
    * Cohort management UI - translated
    * Data download - @explorerleslie should the header row of the student profile CSV be translated? That column is currently translated for the in-page profile info display, but are not for the CSV. For reference, it looks like the grade report header row is not translated either.
  * Forums
    * the cohort selector is translated for thread filtering and when posting new threads
    * "this post is visible to..." is translated

@cahrens @andy-armstrong please review
This is improper use of ngettext. Do not hardcode the 1.

` ngettext('(contains %(student_count)s student)', '(contains %(student_count)s students)', cohort.get('user_count'))`