Add "private: true" to package.json (#3441)

package.json

@@ -17,6 +17,7 @@
     "type": "git",
     "url": "https://github.com/tootsuite/mastodon.git"
   },
+  "private": true,
   "dependencies": {
     "array-includes": "^3.0.3",
     "autoprefixer": "^7.1.0",