post regenerations: replace PostRegeneration model with mod actions.

* Remove the PostRegeneration model. Instead just use a mod action
  to log when a post is regenerated.

* Change it so that IQDB is also updated when the image samples are
  regenerated. This is necessary because when the images samples are
  regenerated, the thumbnail may change, which means IQDB needs to be
  updated too. This can happen when regenerating old images with
  transparent backgrounds where the transparency was flattened to black
  instead of white in the thumbnail.

* Only display one "Regenerate image" option in the post sidebar, to
  regenerate both the images and IQDB. Regenerating IQDB only can be
  done through the API. Having two options in the sidebar is too much
  clutter, and it's too confusing for Mods who don't know the difference
  between an IQDB-only regeneration and a full image regeneration.

* Add a confirm prompt to the "Regenerate image" link.

app/controllers/post_regenerations_controller.rb

@@ -1,18 +1,10 @@
 class PostRegenerationsController < ApplicationController
-  respond_to :html, :xml, :json, :js
+  respond_to :xml, :json, :js
 
   def create
-    @post_regeneration = authorize PostRegeneration.new(creator: CurrentUser.user, **permitted_attributes(PostRegeneration))
-    @post_regeneration.execute_category_action!
-    @post_regeneration.save
+    @post = authorize Post.find(params[:post_id]), :regenerate?
+    @post.regenerate!(params[:category], CurrentUser.user)
 
-    respond_with(@post_regeneration, location: @post_regeneration.post)
-  end
-
-  def index
-    @post_regenerations = authorize PostRegeneration.paginated_search(params)
-    @post_regenerations = @post_regenerations.includes(:creator, :post) if request.format.html?
-
-    respond_with(@post_regenerations)
+    respond_with(@post)
   end
 end