Refactor Comments UI

While working on #1979, I noticed that we happen to register an index page for Comments, but it's hidden by default. This PR updates that UI so it's worthy of being shown by default.

##### Changes
+ new configuration options:
  + menu item visibility (defaults to true)
  + `:as` registration name. For #301; replaces #2060 (defaults to Comment)
+ index page:
  + added a scope for every AA namespace, with the current namespace as the default
  + updated `resource_type` and `author_type` filters to be select fields (for Rails >= 3.2)
  + updated table to show every attribute
+ show page:
  + instead of redirecting to the associated resource, we now have a proper show page