This registers the built-in Gravatar consent rquirement, which will show

up on the My Account page. This is the only consent requirement needed

directly by Review Board. It also includes a custom-made icon for

Gravatar.
This change was meant to go up and land before the previous changes for

the decorator redirects. It fixes test breakages introduced in that

change.

Unit tests pass.
Manually tested the consent requirement and its appearance in the My

Account page.