templatetags: Add fill_url.

This gives templates an easy way to generate full URLs using host_url.

www/conservancy/apps/news/templatetags/fill_url.py

@@ -0,0 +1,20 @@
+import urlparse
+
+from django import template
+
+register = template.Library()
+
+@register.filter(name='fill_url')
+def fill_url(given_url, base_url):
+    """"Fill out" missing pieces of one URL from another.
+
+    This function parses the given URL, and if it's missing any pieces
+    (scheme, netloc, etc.), it fills those in from the base URL.
+    Typical usage is "/URL/path"|fill_url:"https://hostname/"
+    to generate "https://hostname/URL/path".
+    """
+    given_parts = urlparse.urlsplit(given_url)
+    base_parts = urlparse.urlsplit(base_url)
+    return urlparse.urlunsplit(
+        given_part or base_part for given_part, base_part in zip(given_parts, base_parts)
+    )