round avatars everywhere (almost)

fixes CNVS-10713, CNVS-10636

test plan
- ensure that every page that has avatars displays them in a
   consistent, circular way
- ensure that the change didn't break the rest of the page

avatars were left square on purpose on these pages
- speedgrader header (too much work to circle)
- big pic on profile page (too big and interactive, needs
   some proper ui design)
- profile pic selector (not really profile pics yet, just images)
- app review (error handling is tricky)

some pages where you might find avatars to check
- manage avatars (/account/:id/avatars)
- discussions index
- discussions show (don't forget threaded sub-entries!)
- profile page (/about/:id)
- group edit (i can't find this page, but it has code)
- course roster (/courses/:id/users)
- context roster (/groups/:id/users)
- old conversations (message list, message detail, people picker)
- new conversations
- gradebook2 (student column, submission comments)
- speed grader (top left student name, submission comments,
   discussion assignment)
- app review (enable app center plugin, app rating comments)
- user show page (/users/:id)
- masquerade page (/users/:id/masquerade)
- masquerade footer (on the bottom of any page when you masquerade)

Change-Id: Ic01a4b44433aaf6254798d8267bf473a8bf85c2d
Reviewed-on: https://gerrit.instructure.com/28953
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>
QA-Review: Joel Hough <joel@instructure.com>