Fix #4178: add ability to mass undo tag edits.

Adds checkboxes to the /post_versions index allowing you to select and
undo multiple versions at once.

app/models/post_archive.rb

@@ -1,4 +1,5 @@
 class PostArchive < ApplicationRecord
+  class RevertError < Exception ; end
   extend Memoist
 
   belongs_to :post
@@ -218,7 +219,9 @@ class PostArchive < ApplicationRecord
     source.gsub(/^http:\/\//, "").sub(/\/.+/, "")
   end
 
-  def undo
+  def undo!
+    raise RevertError unless post.visible?
+
     added = changes[:added_tags] - changes[:obsolete_added_tags]
     removed = changes[:removed_tags] - changes[:obsolete_removed_tags]
 
@@ -239,11 +242,16 @@ class PostArchive < ApplicationRecord
         post.tag_string = "#{post.tag_string} #{tag}".strip
       end
     end
+
+    post.save!
   end
 
-  def undo!
-    undo
-    post.save!
+  def can_undo?(user)
+    version > 1 && post&.visible? && user.is_member?
+  end
+
+  def can_revert_to?(user)
+    post&.visible? && user.is_member?
   end
 
   def updater_name