The hotly-anticipated sequel of AO3-5694. Now that we've confirmed the current way of aggregating fandoms on user dashboards does not scale, we need an alternative.
We will probably end up using Elasticsearch aggregations (in which case we need to remove the default limit of 10 on the number of facets; related: ).
How to test:
The orphan_account user's dashboard should load and display its fandoms.
Make sure only tags applied directly to the works are included in the list. For example, this means if you post a work and tag it with "Captain America (Movies)", its metatag "Marvel Cinematic Universe" should not appear in your Fandoms box.