Remove envelopes field from category

[morremeyer]

We might be able to remove the envelopes field from the v3.Category. This would make that endpoint much more clean and require fewer database queries.

We need to check:

• Where the envelopes field is used in the frontend
• Provide an alternative if that makes sense
• Remove the field from the v3.Category

If there's endpoints where we definitely need the envelopes, it might be easier to enable querying the backend again with suport for filtering by multiple categories or something similar.

This really is a balance between "make it easy for clients" and "make the backend design as clean as possible"

[morremeyer]

Decided to not do this, the computational overhead is low enough.