Use Bootstrap modal for confirmation of deleting page versions

As an administrator, when deleting a version (or a range of versions) of a page from the history tab, as a confirmation a basic browser alert box is displayed instead of a properly styled modal. 

To reproduce:

Log in as administrator -> view a page -> go to the "History" tab, -> delete a version of a page (or a selected range of versions) by clicking in the "Delete" button (or on the "Delete selected verisons") -> see the alert box.