Use trailing commas where valid in ES5

This will (long-term) reduce noise in out git diffs.

flag=none

Test plan:
  - Automated tests pass

Change-Id: Ie2f52c253e3be005c05bd9c13df7f0536d721089
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/301728
Reviewed-by: Isaac Moore <isaac.moore@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>
Product-Review: Cameron Ray <cameron.ray@instructure.com>

.prettierrc

@@ -3,6 +3,5 @@
   "printWidth": 100,
   "singleQuote": true,
   "bracketSpacing": false,
-  "arrowParens": "avoid",
-  "trailingComma": "none"
+  "arrowParens": "avoid"
 }