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.rb

@@ -753,7 +753,7 @@ class Post < ApplicationRecord
     def filter_metatags(tags)
       @pre_metatags, tags = tags.partition {|x| x =~ /\A(?:rating|parent|-parent|-?locked):/i}
       tags = apply_categorization_metatags(tags)
-      @post_metatags, tags = tags.partition {|x| x =~ /\A(?:-pool|pool|newpool|fav|-fav|child|-child|-favgroup|favgroup|upvote|downvote|status|-status):/i}
+      @post_metatags, tags = tags.partition {|x| x =~ /\A(?:-pool|pool|newpool|fav|-fav|child|-child|-favgroup|favgroup|upvote|downvote|status|-status|disapproved):/i}
       apply_pre_metatags
       return tags
     end
@@ -815,6 +815,10 @@ class Post < ApplicationRecord
           raise User::PrivilegeError unless CurrentUser.is_approver?
           unban!
 
+        when /^disapproved:(.+)$/i
+          raise User::PrivilegeError unless CurrentUser.is_approver?
+          disapprovals.create!(user: CurrentUser.user, reason: $1.downcase)
+
         when /^child:none$/i
           children.each do |post|
             post.update!(parent_id: nil)

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)