The integrations list and configuration pages were always intended to be

usable outside of the Django admin UI, but it was never fully ready for

this. The URL definitions were still built for the admin UI, and so were

the views.
This change makes the integration pages embeddable elsewhere in a

website. A new mixin, IntegrationListContextViewMixin, provides all

the page state needed for rendering the integration configuration list,

and the base views are no longer using the admin templates by default.
Those templates themselves have moved into an integrations/admin/

directory, helping to distinguish them from more reusable templates.
build_integration_urlpatterns() has been updated to take a namespace

for the URLs, allowing for a site to include all the integrations pages

in a way where links will reference the correct places without any risk

of clashing with the admin UI.
There's also a fix for saving integration forms, where the integration

ID could end up unset or could be changed depending on the subclass of

the form. We now force the correct integration ID during saving.

Tested that the admin UI's integration list and all related config pages

still work.
Built a version of the list embedded in another page in its own URL

namespace, and verified that all links work.