Make use of gravatars configurable.

When external internet access is blocked, using Review Board is annoying
because it tries to load gravatars, which can spin for a long time on every
page load, and show broken images. This change adds a setting for whether
gravatars should be used.

Testing done:
Looked at the page header, user infobox, and user page sidebar, and saw that
they all no longer contained broken images.

Fixes bug 3030.

Reviewed at https://reviews.reviewboard.org/r/5656/