Instantiate avatar services form when there is no default service

Review Request #8728 — Created Feb. 8, 2017 and submitted — Latest diff uploaded

Information

Djblets
release-0.10.x

Reviewers

Previously, when the avatar service form was instantiated, it assumed
that there was a default avatar service. If there was not, the form
would fail in instantiation. This patch addresses this.

Ran unit tests. The provided test fails without the fix applied.