Add custom themes, THEME setting, basic "yolocolo" theme (#7)

Specify `THEME=yourtheme`, add some HTML files in `capsulflask/theme/yourtheme` 👌

We probably want to reduce copypasta in the current `yolocolo` theme by using template inheritance, at some point.

Reviewed-on: 3wordchant/capsul-flask#7
Co-authored-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>
Co-committed-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>

capsulflask/theme/yolocolo/support.html

@@ -0,0 +1,21 @@
+{% extends 'base.html' %}
+
+{% block title %}Support{% endblock %}
+
+{% block content %}
+  <div class="row half-margin">
+    <h1>SUPPORT</h1>
+  </div>
+  <div class="row half-margin">
+    <a href="mailto:yolocolo@doesthisthing.work?subject=Please%20help!">yolocolo@doesthisthing.work</a> 
+  </div>
+{% endblock %}
+
+{% block subcontent %}
+<p>
+  You can also find us on Matrix: <a
+    href="https://matrix.to/#/#untitled-hosting.public:autonomic.zone">#untitled-hosting.public:autonomic.zone</a>.
+</p>
+{% endblock %}
+
+{% block pagesource %}/templates/support.html{% endblock %}