add clear cache platform admin page

a form that allows you to clear entries from the cache for all of
either users, services or templates. It'll tell you the largest amount
of keys deleted, since there are multiple keys associated with each
model.

app/templates/views/platform-admin/clear-cache.html

@@ -0,0 +1,21 @@
+{% extends "views/platform-admin/_base_template.html" %}
+{% from "components/form.html" import form_wrapper %}
+{% from "components/radios.html" import radios %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block per_page_title %}
+  Clear Cache
+{% endblock %}
+
+{% block platform_admin_content %}
+
+  <h1 class="heading-large">
+    Clear Redis Cache
+  </h1>
+
+  {% call form_wrapper() %}
+    {{ radios(form.model_type) }}
+    {{ page_footer('Clear') }}
+  {% endcall %}
+
+{% endblock %}