Add `fs-extra` as direct dependency

Allows us to use `fs` with promises. Turns out it’s already a
transitive dependency.

package.json

@@ -62,6 +62,7 @@
     "emoji-panel": "https://github.com/scottnonnenberg/emoji-panel.git#v0.5.5",
     "firstline": "^1.2.1",
     "form-data": "^2.3.2",
+    "fs-extra": "^5.0.0",
     "google-libphonenumber": "^3.0.7",
     "got": "^8.2.0",
     "lodash": "^4.17.4",