Refactor bannerdir to fix import in docs

website/thaliawebsite/templatetags/pick_header_image.py

-from django import template
-from django.contrib.staticfiles.templatetags.staticfiles import static
+import functools
 import os
 import random
 
+from django import template
+from django.conf import settings
+from django.contrib.staticfiles import finders
+
+
 register = template.Library()
-banner_dir = static("/images/header_banners/")
-current_dir = [pic for pic in os.listdir("thaliawebsite/" + banner_dir)
-               if pic.endswith(".jpg")]
+bannerdir = 'images/header_banners'
+
+
+@functools.lru_cache()
+def _banners():
+    imgdir = finders.find(bannerdir)
+    return [pic for pic in os.listdir(imgdir) if pic.endswith('.jpg')]
 
 
 @register.simple_tag
 def pick_header_image():
-    return banner_dir + random.choice(current_dir)
+    """Renders a random header image"""
+    return settings.STATIC_URL + bannerdir + '/' + random.choice(_banners())