Add time of creating of placeholders to placeholders table

Problem

Currently it's not possible to see in UI when each placeholder was created.

Partners working with customers would like to be able to understand which placeholders where created in which "wave" of migration (with Congregate one can migrate in so-called waves, which means a set of groups/projects at a time) to keep a track of it.

Proposed solution

Add a new column to the right of Source column, titled Created at with date-time, in table in Manage > Members > Placeholders tab.

Let's use the shortened format of the date time, like (3 Dec 2024, 17:45:17 CET); based on https://design.gitlab.com/content/date-and-time/

Add this new column (date-time) to the sorting options.