Expose date_of_last_membership_payment in the admin dashboard

[reginafcompton]

Description

This PR accomplishes two related things:

1. date_of_last_membership_payment: replaces add_auto_now with a default value, which makes it a user-editable field. (I believe this will be entirely fine since the project does not expose a public members API and the site does not have a members form that includes this field.
2. expose date_of_last_membership_payment in the admin dashboard

I need this adjustment, since some members overpaid in 2022 (e.g., paid 2023 dues in 2022), and I need to manually update their date_of_last_membership_payment.

Manual Testing

• [ ] create a super user: pipenv run python manage.py createsuperuser
• [ ] login as that user, and go to the admin dashboard: http://127.0.0.1:8000/admin/
• [ ] edit a user; you should see this:

[reginafcompton]

@ecedmondson - do you have time to look at this PR in next day or so? It should be fairly straightforward...but maybe I missed something silly in the migration/model adjustment.

[ecedmondson]

Sure. I just had this notification come through. I can't do it now but I will get to it soon.

[ecedmondson]

. (I believe this will be entirely fine since the project does not expose a public members API and the site does not have a members form that includes this field.

This seems probably fine. If you're ever worried about it you can subclass save and log details if that particular field is being updated?