Bugfix/2363 with main item selection

https://www.openproject.org/issues/2363

This branch is based on bugfix/2363_default_wiki_menu_item and redefines the specification.

Formerly - in bugfix/2363_default_wiki_menu_item - when the last main item is deleted, a new main item is automatically created for another main wiki page.

In this branch the user deleting the last main wiki menu item is instead given the choice to select another wiki page for which a menu entry is then created.