We previously styled avatars differently in different parts of the UI.

Most places would style the avatars to be rounded, but some places would

not, and it entirely depended on where the avatar was.
This adds some common styling that gives avatars a rounded look by

default. Places that need to show a square view (such as the infobox)

can (and currently do) override the border-radius to remove the

rounded look.

Looked around the UI for all places where we use avatars, both in

Review Board and RBCommons, checking that they appeared as expected.