Move dropzone.js to npm/webpack (#10645)

- unvendor dropzone and upgrade it from 4.2.0 to 5.7.0
- make `csrf` available on window.config

package.json

@@ -7,6 +7,7 @@
   "dependencies": {
     "@primer/octicons": "9.4.0",
     "clipboard": "2.0.4",
+    "dropzone": "5.7.0",
     "fomantic-ui": "2.8.4",
     "highlight.js": "9.18.1",
     "jquery": "3.4.1",