Add Ruffle support for Flash files.

Allow viewing Flash posts with the Ruffle emulator.

Known issues:

* Many flash files aren't fully supported.
* In development it sometimes spazzes out and starts triggering random
  keyboard shortcuts when you press any key. This doesn't happen with
  the browser extension.
* We have to put the .wasm file in the public/packs/js directory because
  Ruffle is hardcoded to search for it there.
* If you're running Nginx, you need to make sure you're serving the
  right MIME type for .wasm files or it won't work.
* We're using Some Random Guy's unofficial NPM package for Ruffle, since the
  Ruffle project doesn't publish an official package themselves. We
  should build our own package.

References:

* https://github.com/ruffle-rs/ruffle
* https://github.com/ruffle-rs/ruffle/wiki/Using-Ruffle#configure-webassembly-mime-type
* https://www.npmjs.com/package/ruffle-mirror

app/views/posts/partials/show/_flash.html.erb

@@ -4,3 +4,5 @@
 <% end %>
 
 <p><%= link_to "Save this flash (right click and save)", post.tagged_file_url %></p>
+
+<%= javascript_pack_tag "flash" %>