Remove dSS and use dSS2 on the overview page.

Also remove all of the crufty files that referenced dSS.

Refs #11781.
Change disableSingletonDropdown to run on DOM ready

Refactored the code to be more clear, and removed unnecessary deletion of page
elements

select element's CSS and ID now gets copied over to new element

refs #enhancement/11781
Updated docs to say that we don't support Ganeti<2.3

Refs #11319
Merge branch 'bug/11319' into develop

* bug/11319:
  Updated docs to say that we don't support Ganeti<2.3
Merge branch 'enhancement/11781' into develop

Conflicts:
	ganeti_web/static/js/disableSingletonDropdown.js
	ganeti_web/templates/ganeti/forms/vm_wizard.html

This removes all references to old disableSingletonDropdown function.