* DEV: small refactor of the category_moderators method
Used `index_by(&:id)` instead of `map { |u| [u.id, u] }.to_h` thanks to @cvx's recommendation.
Also renamed the `moderators` variable to not clash with method of the same name.
In the about page, we list a certain number of category moderators.
This rewrites the SQL query used to retrieve the most recent category moderators in order
to perform better with a large number of users/categories/category moderators.
TIL: you can ORDER BY inside an ARRAY_AGG in postgres
TIL: you can slide ARRAYS in postgres
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
