Rename "Recent Activity Dashboard"

On the "Choose Home Page" modal, the
option "Recent Activity Dashboard" has
been renamed to "Course Activity Stream".

closes CNVS-16582

Test Plan:
* As a teacher or admin, go to any course's homepage
* Click the "Choose Home Page" button on the right sidebar
* The first option should say "Course Activity Stream"

Change-Id: Iea2dda10c209c546ccaf84de9b187f6b8caf55c1
Reviewed-on: https://gerrit.instructure.com/44304
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>