Add routing plan

Signed-off-by: Zeljko <zeljko@zwr.fi>
Admin namespace

All scaffolds are now in the Admin namespace, which also
has its own layout:

- Admin::AdminController is the parent controller (still
  itself comes from ApplicationController, so that one can
  be used for generic behaviour on both sides)
- All admin controllers and views are in admin subfolders
  of their folders (controllers and views in app)
- Views are not changed, controllers have changed parent
- Controller tests for scaffolds are moved themselves into
  Admin namespace, so they can easily find their controllers.
- Routes changed

About routes: setting 'scope', 'module' and then changing
the default helpers with 'as:' parameter was the only way
I figure out to make _path and _url helpers stay without
the admin_ prefix. Otherwise changing namespace would imply
changing code all around the app, which is not acceptable.

Signed-off-by: Zeljko <zeljko@zwr.fi>