Avoid N+1 query in Tag#tagnames_as_classes

I noticed that `Tag#tagnames_as_classes` performs a `Node.find(id)` call,
which can lead to N+1 queries when rendering tag lists.

Since `Tag` already has a `node` association, this can be rewritten to
use the association and allow eager loading with `.includes(:node)`.

This would improve performance without changing behavior.

I’d like to work on this if that’s okay.
