posts: add disapproved:<reason> edit metatag.

* Allow tagging a post with a `disapproved:<disinterest|breaks_rules|poor_quality>` to disapprove it.
* Disallow disapproving active posts.

Fixes #4384.

app/models/post_disapproval.rb

@@ -6,6 +6,7 @@ class PostDisapproval < ApplicationRecord
   belongs_to :user
   validates_uniqueness_of :post_id, :scope => [:user_id], :message => "have already hidden this post"
   validates_inclusion_of :reason, in: REASONS
+  validate :validate_disapproval
 
   scope :with_message, -> { where.not(message: nil) }
   scope :without_message, -> { where(message: nil) }
@@ -62,6 +63,12 @@ class PostDisapproval < ApplicationRecord
     [:user, :post]
   end
 
+  def validate_disapproval
+    if post.status == "active"
+      errors[:post] << "is already active and cannot be disapproved"
+    end
+  end
+
   def message=(message)
     message = nil if message.blank?
     super(message)