post replacement: take guidelines from help:replacement_notice.

app/views/post_replacements/_new.html.erb

@@ -1,11 +1,5 @@
+<%= format_text(WikiPage.titled(Danbooru.config.replacement_notice_wiki_page).first.try(&:body), ragel: true) %>
+
 <%= simple_form_for(post_replacement, url: post_replacements_path(post_id: post_replacement.post_id), method: :post) do |f| %>
-  <h1>Replace Image</h1>
-
-  <p>
-    Delete the current image and replace it with another one, keeping
-    everything else in the post intact. This is meant for upgrading
-    lower-quality images, such as image samples, to higher-quality versions.
-  </p>
-
   <%= f.input :replacement_url, label: "New Source", input_html: { value: "" } %>
 <% end %>

config/danbooru_default_config.rb

@@ -271,6 +271,10 @@ module Danbooru
       "help:appeal_notice"
     end
 
+    def replacement_notice_wiki_page
+      "help:replacement_notice"
+    end
+
     # The number of posts displayed per page.
     def posts_per_page
       20