[Slack][Membership Command] Use static selects for MembershipOptions, TrainableEquipment

[chungl]

We should use static selects in our Slack modals rather than external selects whenever possible. Obvious candidates are the initial MembershipOptionsModal and the equipment list in EquipmentAuthorization.

External selects require slack to send a network request to webhooks to fetch the list of options, and are useful for lists that may change or are too large to prepopulate. However, this introduces noticeable latency to the modal as the user has to wait for select menus to populate. The static select element allows us to send the options as part of the view update, eliminating this round trip and improving UX.