tests(fxa-client): add an fxa-client mock
fix(avatars): redirect unverified users to confirm screen

Fixes #1662.
tests(fxa-client): create mock based on the original client prototype
Merge pull request #1697 from mozilla/issue-1662-avatar-unverified-error

fix(avatars): redirect unverified users to confirm screen

@zaach - this is awesome, I love the FxaClient Mock.

r+