Simplify MemberView by fixing size instead of calculating it dynamically

Gijs Hendriksenrequested to merge
fix/member-view into master

Short description

Simplifies the MemberView and SquareView by giving it a fixed size instead of calculating it upon layout. This makes it easier to show a list of members, for example with an event with registrations, as you can now simply use a FlatList with 3 columns.

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