The Djblets siteconfig app builds on the admin `change_form.html` template, but changes to that template were made at some point which were causing problems. Specifically, Django seems to have added the ability to override that template on a per-app or per-model basis, but that assumed that it was being rendered from within the admin view, which inserted an `opts` into the render context which was the model's `_meta` object. This change adds the siteconfig's `_meta` object in appropriately so that Django's internal `InclusionAdminTag` doesn't explode. Testing Done: Was able to open various settings pages in the Review Board admin UI without it exploding.