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/crypto/encrypted_messages_controller.rb

@@ -26,7 +26,7 @@ class Api::V1::Crypto::EncryptedMessagesController < Api::BaseController
   end
 
   def set_encrypted_messages
-    @encrypted_messages = @current_device.encrypted_messages.paginate_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
+    @encrypted_messages = @current_device.encrypted_messages.to_a_paginated_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
   end
 
   def insert_pagination_headers