Fixes#9842.
Enables avatar images in pills wherever user_pill.js is used.
(e.g composebox, user group settings)
Changes to search_pill.js are not made as search pills haven't been
added yet completely and search_pill.js just contains the preparatory
code right now.
No change to compose_pm_pill.js is not required as it uses
`user_pill.create_item_from_text` in its `create` function.
This run_test helper sets up a convention that allows
us to give really short tracebacks for errors, and
eventually we can have more control over running
individual tests. (The latter goal has some
complications, since we often intentionally leak
setup in tests.)
Shubham Padia
Joshua Pan
Steve Howell