[UX] Allow reordering of layouts from main Layouts page

This is a follow-on to https://github.com/backdrop/backdrop-issues/issues/5282#issuecomment-932...

• The current path-specific layout reordering page has weird URLs (due to passing much information as query parameters) that makes the page fragile and easily broken (see, for example, 2nd case in https://github.com/backdrop/backdrop-issues/issues/5282#issuecomment-998...).
• It would be more user-friendly to allow users to reorder layouts on the main Layouts page, rather than making them click through to a sub-page for each path.
• Reordering on the main page would make reorderability more discoverable to new users (currently one has to know to explore the dropbuttons to find out that reordering is a possibility).