create proposal (not amendment) from classic norm
fixup: link new proposal to page
check page.allow_selection for amendments in proposal.new
allow selection only if selections are enabled in instance or if the page is a sectionpage
show page option allow_selection only if not blocked by instance

Not sure at all if this is how it should work
Especially as the same option also shows up in page/new
I left it there because it is not clear at that time if the page
is a sectionpage.
Merge pull request #660 from xi/feature-page-proposals

create proposal (not amendment) from classic norm