Dashboard sidebars retrieve work and bookmark counts from Elasticsearch, which creates long running queries especially on users or pseuds with a lot of works and bookmarks. We need to cache these counts. The cache time should be configurable.
Series counts come from the database and they're not as much of a problem, but let's cache them anyway. See AO3-5829.
Note: We've added a workaround in nginx that renders /users/orphan_account/pseuds/* pages as 404. We can remove that once caching is in place.
How to test:
Check that the dashboard work count is eventually updated after a work is created / deleted / posted from draft / (un)revealed.
Check that the dashboard bookmark count is eventually updated after a bookmark is created / deleted / made private / made public.