Bug 1008447: Add CachingMixin and CacheManager to User class.

When a new link is created, the cached results for a user’s links
doesn’t get invalidated because, while the link has a foreign key to the
user, the user isn’t cached by django-cache-machine. By adding the mixin
and manager to the user class, when a new link is created, the user, and
thus it’s related links, should be invalidated, fixing the caching 
issues on the dashboard.
Avoid count query in stats display to avoid caching.

Since all the pages the stats are included on already use the user’s
link_set, the results are cached (yet up to date due to invalidation) 
and don’t add any queries.
Styling for old versions of IE