sources: remove image_url method from base strategy.

Remove the `image_url` method from source strategies. This method would
return only the first image if a source had multiple images. The
`image_urls` method should be used instead. Tests were the main place
that still used `image_url` instead of `image_urls`.

Also make post replacements return an error if replacing with a source
that contains multiple images, instead of just blindly replacing the
post with the first image in the source.

test/test_helpers/download_test_helper.rb

@@ -1,13 +1,13 @@
 module DownloadTestHelper
   def assert_downloaded(expected_filesize, source, referer = nil)
     strategy = Sources::Strategies.find(source, referer)
-    file = strategy.download_file!
+    file = strategy.download_file!(strategy.image_urls.sole)
     assert_equal(expected_filesize, file.size, "Tested source URL: #{source}")
   end
 
   def assert_rewritten(expected_source, test_source, test_referer = nil)
     strategy = Sources::Strategies.find(test_source, test_referer)
-    rewritten_source = strategy.image_url
+    rewritten_source = strategy.image_urls.sole
     assert_match(expected_source, rewritten_source, "Tested source URL: #{test_source}")
   end