Issue 1459 avatar functional tests

This depends on #1512 (only the last commit is for this PR). This mocks out some client side functionality, like capturing webcam images and selecting a file from the filesystem.

Fixes #1459.
Fixes #1494.
Just like https://github.com/mozilla/fxa-content-server/pull/1532/files#r16092762 
but maybe use a tiny data URI image that we can rotate ?