Remove will_paginate gem, use kaminari as already required by rails admin
Switch rails_admin to use paper_trail gem instead of built-in history
Basic multi-tenancy view of rails admin interface

This commit is mostly to make it easier to visualize what information is
associated with the site and auto-fill some form fields (not security).

All models that support tenantization should now have the 'site' field
set to the current site's site association automatically.

The rails admin interface will also only show model objects that pertain to
the site being viewed unless they're considered to be cross-site model.

History views in rails admin currently shows history across all sites as
paper_trail hasn't been set up for tenantization yet.