Fix time translations in course people page

fixes VICE-2095
flag=none

Test Plan:
 - ensure I18n plugin is enabled with custom Spanish translations
 - go to course settings
 - change language to a custom Spanish setting
      Example: Español (Chile)
 - go to the course people page
 > last activity should have the format “{date} en {time}”

Note:
I am having a hard time reproducing in my local dev environment.
I am getting a result that is similar when setting my language to
non-custom Espanol. We will need to verify that this works on CD
to be sure.

Change-Id: I4203af22816ff3931a854fd7ef13dba9e2e717e2
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275011
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

ui/features/roster/jst/rosterUser.handlebars

@@ -40,7 +40,7 @@
 <td>
   {{#each enrollments}}
     <div {{contextSensitiveDatetimeTitle last_activity_at }}>
-      {{datetimeFormatted last_activity_at}}
+      {{tEventToString last_activity_at 'short' 'tiny'}}
     </div>
   {{/each}}
 </td>