Move post replacement create action to post replacements controller.

app/views/post_replacements/_new.html.erb

@@ -0,0 +1,11 @@
+<%= 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 %>

app/views/post_replacements/new.html.erb

@@ -0,0 +1,5 @@
+<div id="c-post-replacements">
+  <div id="a-new">
+    <%= render "new", post_replacement: @post.replacements.new %>
+  </div>
+</div>