Downgrade JSZip from 3.5.0 to 3.1.5.

Performance issues as documented here:
Stuk/jszip#617

Author: funkyhippo

reader/templates/reader/reader.html

@@ -154,7 +154,7 @@ <h1 data-bind='title'><a href="/read/manga/{{ slug }}/">{{ series_name }}</a></h
   </div>
 </body>
 <script src="https://cdn.jsdelivr.net/npm/@simonwep/pickr/dist/pickr.min.js"></script>
-<script src="https://cdn.jsdelivr.net/npm/jszip@3.5.0/dist/jszip.min.js"></script>
+<script src="https://cdn.jsdelivr.net/npm/jszip@3.1.5/dist/jszip.min.js"></script>
 <script type="text/javascript" src="{% static 'js/ResizeSensor.js' %}{{ version_query }}"></script>
 <script type="text/javascript" src="{% static 'js/alg_lib.js' %}{{ version_query }}"></script>
 <script type="text/javascript" src="{% static 'js/UI2.js' %}{{ version_query }}"></script>