This change resolves#3512 and #3636 by removing 'magic' link generation
and adding an extension method to add routes to areas correctly using the new
pattern. This is pretty much exactly the same as how MapWebApiRoute works.
For site authors, we recommend adding area-specific routes in a way that
includes a default AND constraint for the area. Put your most specific
(for link generation) routes FIRST.
Ex:
routes.MapRoute(
"Admin/{controller}/{action}/{id?}",
defaults: new { area = "Admin" },
constraints: new { area = "Admin" });
The bulk of the changes here are to tests that unwittingly relied on the
old behavior.
