Add linter for JS files with basic recommended rules:

- Add Github Action.
- Fix JS files to match rules; mostly `globals` and `exported`.

.github/workflows/linters-frontend.yaml

@@ -22,3 +22,9 @@ jobs:
         uses: actions-hub/stylelint@v1.1.3
         env:
           PATTERN: "*.css"
+
+      - name: Run ESLint
+        uses: stefanoeb/eslint-action@1.0.2
+        with:
+          files:
+            - 'bookwyrm/static/js/**'