Refactor notifiable_available? method

This method was calling `check_availability`, which returned a boolean
value and caused a `Naming/PredicateMethod` when upgrading rubocop.

So we're changing the logic a little bit to remove the
`check_availability` method and merge the tests of `check_availability`
and `notifiable_available?` (which were almost identical) together.

app/models/concerns/notifiable.rb

@@ -17,20 +17,20 @@ module Notifiable
   end
 
   def notifiable_available?
-    case self.class.name
-    when "ProposalNotification"
-      check_availability(proposal)
-    when "Comment"
-      check_availability(commentable)
-    else
-      check_availability(self)
-    end
+    notifiable_resource.present? &&
+      !(notifiable_resource.respond_to?(:hidden?) && notifiable_resource.hidden?) &&
+      !(notifiable_resource.respond_to?(:retired?) && notifiable_resource.retired?)
   end
 
-  def check_availability(resource)
-    resource.present? &&
-      !(resource.respond_to?(:hidden?) && resource.hidden?) &&
-      !(resource.respond_to?(:retired?) && resource.retired?)
+  def notifiable_resource
+    case self.class.name
+    when "ProposalNotification"
+      proposal
+    when "Comment"
+      commentable
+    else
+      self
+    end
   end
 
   def linkable_resource

app/models/notification.rb

@@ -10,8 +10,7 @@ class Notification < ApplicationRecord
   scope :recent,      -> { order(id: :desc) }
   scope :for_render,  -> { includes(:notifiable) }
 
-  delegate :notifiable_title, :notifiable_body, :notifiable_available?,
-           :check_availability, :linkable_resource,
+  delegate :notifiable_title, :notifiable_body, :notifiable_available?, :linkable_resource,
            to: :notifiable, allow_nil: true
 
   def mark_as_read