models: refactor search visibility methods.

Refactor how model visibility works in index actions:

* Call `visible` in the controller instead of in model `search`
  methods. This decouples model visibility from model searching.

* Explicitly pass CurrentUser when calling `visible`. This reduces
  hidden dependencies on the current user inside models.

* Standardize on calling the method `visible`. In some places it was
  called `permitted` instead.

* Add a `visible` base method to ApplicationModel.

app/controllers/dmails_controller.rb

@@ -15,7 +15,7 @@ class DmailsController < ApplicationController
   end
 
   def index
-    @dmails = Dmail.visible.paginated_search(params, count_pages: true)
+    @dmails = Dmail.visible(CurrentUser.user).paginated_search(params, count_pages: true)
     @dmails = @dmails.includes(:owner, :to, :from) if request.format.html?
 
     respond_with(@dmails)