LPS-30833 Site pages from site template are neither sortable nor parentable

Hey Julio,

@JorgeFerrer and I decided to lock the sorting and nesting operations over pages created from site templates to prevent unexpected behaviours when reseting and propagating changes from the site template, as described in LPS-30833.

@ipeychev has already reviewed the frontend, but maybe you want to send it to @natecavanaugh, too.

Thnx!