We'd previously import and construct the avatar and search registries

when loading the reviewboard.avatars and reviewboard.search apps for

the first time. In modern versions of Django, this breaks app

initialization, since the imports end up importing other uninitialized

apps, resulting in hard-to-diagnose error messages.
To work around this, we now lazily import and construct the registries

when they're first needed. This has a side benefit of reducing the

startup load, even if just a bit. To do this, we use the new

lazy_import_registry() function in Djblets, which will construct a

wrapper around on the registries that does the import and construction

on first access, which will be some time after Django initialization has

completed.

Unit tests pass on Django 1.6 and 1.11 (with other in-progress changes).