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: #7
Co-authored-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>
Co-committed-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>

capsulflask/theme/yolocolo/pricing.html

@@ -0,0 +1,23 @@
+{% extends 'base.html' %}
+
+{% block title %}Pricing{% endblock %}
+
+{% block content %}
+  <div class="row third-margin">
+    <h1>CAPSUL TYPES & PRICING</h1>
+  </div>
+  <div class="row half-margin">
+    <p>
+      Rates for this service aren't set yet. You can see Cyberia's Capsul pricing
+      on <a href="https://capsul.org/pricing">their website</a>.
+    </p>
+  </div>
+  <div>
+    <pre>
+      SUPPORTED OPERATING SYSTEMS:
+
+      {% for os_id, os in operating_systems.items() %}   - {{ os.description }} 
+      {% endfor %}
+    </pre>
+  </div>
+{% endblock %}