replace "Rich Text Editor" with "Rich Content Editor"

Test Plan:
  - create or edit a quiz
  - switch editors by clicking on "HTML Editor"
  - Confirm that the editor switch text now reads 'Rich Content Editor'.
  Extra WYSIWYG locations:
  - Confirm that assignments, syllabus content, calendar events, eportfolios, and wiki rich text editors are also edited
  - confirm that locales are updated

Closes #CNVS-14658

Change-Id: I87348365706cd7a9d77932b805204879e521425d
Reviewed-on: https://gerrit.instructure.com/38885
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>