We had hacks around the templates for falling back on defaults if

settings were missing from siteconfig. This was missing in some cases,

and resulted in breakages in the case of displaying gravatars (a recent

change).
Djblets now provides siteconfig_settings, which allows access with

falling back on defaults. This change updates all templates to use it.

Tested each of these conditionals.
Saw that gravatars were working again, without having the setting in

my siteconfig.