Member list category benefactors

One-sentence description

Add benefactors as category to member list.

Motivation

Currently it's inconsistent. There is a category 'honorary members' and even 'old members' but no 'benefactors' whereas it really is a separate group Thalians.

Desired functionality

Also have a tab 'benefactors' at the members list page.

Suggested implementation

I think this is quite clear.

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