Make Array-creation behavior of Paginable more predictable (#14687)

* Make Array-creation behavior of Paginable more predictable

Paginable.paginate_by_id usually returns ActiveRecord::Relation, but it
returns an Array if min_id option is present. The behavior caused problems
fixed with the following commits:
- 552e886b64
- b63ede5005
- 64ef37b89d

To prevent from recurring similar problems, this commit introduces two
changes:
- The scope now always returns an Array whether min_id option is present
  or not.
- The scope is renamed to to_a_paginated_by_id to clarify it returns an
  Array.

* Transform Paginable.to_a_paginated_by_id from a scope to a class method

https://api.rubyonrails.org/classes/ActiveRecord/Scoping/Named/ClassMethods.html#method-i-scope
> The method is intended to return an ActiveRecord::Relation object, which
> is composable with other scopes.

Paginable.to_a_paginated_by_id returns an Array and is not appropriate
as a scope.

app/controllers/api/v1/admin/accounts_controller.rb

@@ -79,7 +79,7 @@ class Api::V1::Admin::AccountsController < Api::BaseController
   private
 
   def set_accounts
-    @accounts = filtered_accounts.order(id: :desc).includes(user: [:invite_request, :invite]).paginate_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
+    @accounts = filtered_accounts.order(id: :desc).includes(user: [:invite_request, :invite]).to_a_paginated_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
   end
 
   def set_account

app/controllers/api/v1/admin/reports_controller.rb

@@ -63,7 +63,7 @@ class Api::V1::Admin::ReportsController < Api::BaseController
   private
 
   def set_reports
-    @reports = filtered_reports.order(id: :desc).with_accounts.paginate_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
+    @reports = filtered_reports.order(id: :desc).with_accounts.to_a_paginated_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
   end
 
   def set_report