Add eslint command to yarn (CTFd#1324)

* Adds a `yarn lint` command that runs `eslint`
* Starts work on CTFd#1245

Author: ColdHeat

.eslintrc.js

@@ -0,0 +1,17 @@
+module.exports = {
+    "env": {
+        "browser": true,
+        "es6": true
+    },
+    "extends": "eslint:recommended",
+    "globals": {
+        "Atomics": "readonly",
+        "SharedArrayBuffer": "readonly"
+    },
+    "parserOptions": {
+        "ecmaVersion": 2018,
+        "sourceType": "module"
+    },
+    "rules": {
+    }
+};

package.json

@@ -10,7 +10,8 @@
     "dev": "node_modules/webpack-cli/bin/cli.js -w --mode=development",
     "build": "node_modules/webpack-cli/bin/cli.js --mode=development; node_modules/webpack-cli/bin/cli.js --mode=production",
     "verify": "node_modules/webpack-cli/bin/cli.js --mode=development --display=errors-only; node_modules/webpack-cli/bin/cli.js --mode=production --display=errors-only; git diff --quiet --exit-code",
-    "clean": "rm -rf CTFd/themes/core/static/css/* CTFd/themes/core/static/js/* CTFd/themes/admin/static/css/* CTFd/themes/admin/static/js/*"
+    "clean": "rm -rf CTFd/themes/core/static/css/* CTFd/themes/core/static/js/* CTFd/themes/admin/static/css/* CTFd/themes/admin/static/js/*",
+    "lint": "node_modules/.bin/eslint CTFd/themes/core/assets/ CTFd/themes/admin/assets/"
   },
   "repository": {
     "type": "git",