Externalize the display of the wikis list in the farm importer to allow to easily overwrite it if a different UI needs to be displayed for selection

In the farm importer, when many wikis are to be upgraded and selected from the list, the selection of the wikis on which to apply / import can be cumbersome.

The idea of this task is to externalise the code that displays the wikis list from the scripts, and isolate it in a separate document, so that this separate document can be easily customized to display a custom selector for the wikis, based on various elements that are specific.