Make some terms consistent

System menu, Start menu->User menu
Program->App
[endlessm/eos-shell#3910]
Remove reference to deja dup

I don't think we ship that application
[endlessm/eos-shell#3910]
Restructure page hierarchy

We now have a much simpler page hierarchy with 6 toplevel pages
which link to all the article pages.

Removed see also links
[endlessm/eos-shell#3910]
Fix broken links in help text

There were a lot of links to pages that no longer exist in pages we brought
back from the gnome-3-10 branch
[endlessm/eos-shell#3910]
Content revisions from code review
[endlessm/eos-shell#3910]