This changes UserReferenceFilter so it operates using the following
steps:

1. Grab all username references from the input document.
2. Query the corresponding Namespace objects using a single query.
3. Iterate over all nodes to build links while re-using the objects
   queried in step 2.

The impact of these changes is that a comment mentioning 5 different
usernames no longer runs 5 different queries (1 for every username),
instead it only runs a single query.