post replacements: add option to fix source after replacement.

Adds a "Final Source" field to the post replacement dialog. If
specified, the post's source field will be changed to this value after
replacement.

This makes fixing the source back to the HTML page after
replacement easier.

test/unit/post_replacement_test.rb

@@ -190,5 +190,15 @@ class PostReplacementTest < ActiveSupport::TestCase
         assert(File.exists?(@post.large_file_path))
       end
     end
+
+    context "a post when given a final_source" do
+      should "change the source to the final_source" do
+        replacement_url = "http://data.tumblr.com/afed9f5b3c33c39dc8c967e262955de2/tumblr_orwwptNBCE1wsfqepo1_raw.png"
+        final_source = "https://noizave.tumblr.com/post/162094447052"
+        @post.replace!(replacement_url: replacement_url, final_source: final_source)
+
+        assert_equal(final_source, @post.source)
+      end
+    end
   end
 end