refactor(avatars): add an image loading helper
fix(avatars): show a spinner icon when loading images with latency

This refactors the button progress indicator into something more generic
and uses that to implement progress indicators for profile image loading.

Fixes #1527.
Merge pull request #1666 from mozilla/issue-1527-avatar-load-transition

fix(avatars): show a spinner icon when loading images with latency

fixes #1527
r=ckarlof