post replacements: add "Tags" field to dialog box.

* Adds a "Tags" field to the post replacement dialog box. The given tags
  are added to the post after replacement.

* Prefills the Tags field with certain tags that usually need to be
  removed after replacement: replaceme, image_sample, jpeg_artifacts, etc.

app/models/post_replacement.rb

@@ -4,11 +4,12 @@ class PostReplacement < ApplicationRecord
   belongs_to :post
   belongs_to :creator, class_name: "User"
   before_validation :initialize_fields
-  attr_accessor :replacement_file, :final_source
+  attr_accessor :replacement_file, :final_source, :tags
 
   def initialize_fields
     self.creator = CurrentUser.user
     self.original_url = post.source
+    self.tags = post.tag_string + " " + self.tags.to_s
   end
 
   def undo!
@@ -23,7 +24,7 @@ class PostReplacement < ApplicationRecord
     end
 
     transaction do
-      upload = Upload.create!(file: replacement_file, source: replacement_url, rating: post.rating, tag_string: post.tag_string)
+      upload = Upload.create!(file: replacement_file, source: replacement_url, rating: post.rating, tag_string: self.tags)
       upload.process_upload
       upload.update(status: "completed", post_id: post.id)
 
@@ -155,6 +156,12 @@ class PostReplacement < ApplicationRecord
         truncated_source
       end
     end
+
+    def suggested_tags_for_removal
+      tags = post.tag_array.select { |tag| Danbooru.config.remove_tag_after_replacement?(tag) }
+      tags = tags.map { |tag| "-#{tag}" }
+      tags.join(" ")
+    end
   end
 
   include PresenterMethods