[assets] Remove redundant class already provided by bulma:

Replace all classes `hidden` with `is-hidden` in templates, CSS and JS.

bookwyrm/templates/user/lists.html

@@ -24,7 +24,7 @@
 
 {% block panel %}
 <section class="block content">
-    <form name="create-list" method="post" action="{% url 'lists' %}" class="box hidden" id="create-list">
+    <form name="create-list" method="post" action="{% url 'lists' %}" class="box is-hidden" id="create-list">
         <header class="columns">
             <h3 class="title column">{% trans "Create list" %}</h3>
             <div class="column is-narrow">