Organize admin menu

A number of admin pages try to insert themselves in the menu at a particular spot but fail to do so. For example, Dashboard registers itself as the 1st menu item, Pages as the 2nd, and Content as the 3rd:

sec/app/admin/dashboard.rb

Line 3 in 4a3bd36

menu priority: 1, label: proc{ I18n.t("active_admin.dashboard") }

sec/config/initializers/active_admin.rb

Line 301 in f72ed2e

menu.add label: "Pages", priority: 2 do |pages|

sec/config/initializers/active_admin.rb

Line 297 in f72ed2e

menu.add label: "Content", priority: 3

But instead, Pages is the last menu item

It seems confusing and bug prone to hard code priories across separate files like this. It would be great if we could define the menu link sequence all in one place.

The pages menu in particular has some other quirks.

Homepage appears in the menu but not in the pages index table.
Articles Overview is linked under Pages, but if you navigate to it indirectly through the index table, you get a different form that doesn't allow full editing of the page.
I think ideally the overview links should be grouped together, but right now they're split by a link to Credits.

A fix for these issues could be not exposing the pages index and only allowing navigation to them through the menu. That menu should start with Homepage, then the Overview pages, and then an alphabetical list of the other pages.