[assets] Move some frontend assets to _vendor_ directories:

This simplifies linting of files we have no grasp on, and clarifies responsibilities.

- Add .eslintignore.
- Restrict linting to bookwyrm/static.

bookwyrm/templates/book/book.html

@@ -267,5 +267,5 @@
 {% endblock %}
 
 {% block scripts %}
-<script src="/static/js/tabs.js"></script>
+<script src="/static/js/vendor/tabs.js"></script>
 {% endblock %}