Move post replacement create action to post replacements controller.

app/views/moderator/post/posts/_replace.html.erb

@@ -1,11 +0,0 @@
-<%= simple_form_for(@post, url: replace_moderator_post_post_path, 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 :source, label: "New Source", input_html: { value: "" } %>
-<% end %>

app/views/moderator/post/posts/replace.html.erb

@@ -1,5 +0,0 @@
-<div id="c-moderator-post-posts">
-  <div id="a-replace">
-    <%= render "moderator/post/posts/replace" %>
-  </div>
-</div>