models: factor out api_attributes to policies.

Refactor models so that we define attribute API permissions in policy
files instead of directly in models.

This is cleaner because a) permissions are better handled by policies
and b) which attributes are visible to the API is an API-level concern
that models shouldn't have to care about.

This fixes an issue with not being able to precompile CSS/JS assets
unless the database was up and running. This was a problem when building
Docker images because we don't have a database at build time. We needed
the database because `api_attributes` was a class-level macro in some
places, which meant it ran at boot time, but this triggered a database
call because api_attributes used database introspection to get the list
of allowed API attributes.

app/policies/application_policy.rb

@@ -69,4 +69,8 @@ class ApplicationPolicy
   def permitted_attributes_for_edit
     permitted_attributes_for_update
   end
+
+  def api_attributes
+    record.class.attribute_types.reject { |name, attr| attr.type.in?([:inet, :tsvector]) }.keys.map(&:to_sym)
+  end
 end