fix: restore conf.Get().AppURL but still fall back to globals.AppURL
Created by: chrismwendt
I had mistakenly removed the call to conf.Get().AppURL
in https://github.com/sourcegraph/sourcegraph/pull/355/files#diff-46349eab2d451601271f4863de4b3cf9L129 which prevented setting appURL
in site config from having any effect.
This PR does not need to update the CHANGELOG because this fixes an unreleased bug.