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/models/account_conversation.rb

@@ -36,11 +36,11 @@ class AccountConversation < ApplicationRecord
   end
 
   class << self
-    def paginate_by_id(limit, options = {})
+    def to_a_paginated_by_id(limit, options = {})
       if options[:min_id]
         paginate_by_min_id(limit, options[:min_id]).reverse
       else
-        paginate_by_max_id(limit, options[:max_id], options[:since_id])
+        paginate_by_max_id(limit, options[:max_id], options[:since_id]).to_a
       end
     end