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

yarn.lock

@@ -8340,6 +8340,7 @@ fsevents@~2.3.1:
     postcss-loader: ^5.0.0
     postcss-preset-env: ^6.7.0
     rails-erb-loader: ^5.5.0
+    ruffle-mirror: ^2021.3.31
     sass: ^1.32.5
     sass-loader: ^11.0.0
     spark-md5: ^3.0.0
@@ -8353,6 +8354,13 @@ fsevents@~2.3.1:
   languageName: unknown
   linkType: soft
 
+"ruffle-mirror@npm:^2021.3.31":
+  version: 2021.3.31
+  resolution: "ruffle-mirror@npm:2021.3.31"
+  checksum: 2247ab90331e9e9a6c532a576ef04f31ecde95fe246b83f96f3af1facd4ab566e99e7834ddfbe8ea5932594e187c78515bac174382c630e34a7ac72b7d6bbb8e
+  languageName: node
+  linkType: hard
+
 "run-parallel@npm:^1.1.9":
   version: 1.2.0
   resolution: "run-parallel@npm:1.2.0"