[VMS] Add a supporters array to nonprofit schema

[PC98]

• We want to keep track of who supports (users that have donated or volunteered at) a non-profit. We could get this information using the existing events and donations field of each non-profit, but it will be cumbersome to get unique user IDs from all events and donations of a non-profit. Before doing this task, if you can figure out an easy way to get unique user IDs from these two fields, lmk and we won't have to do this.
• Add a supporters field - array of user IDs - to the non-profits schema. Write a migration script as well.
• Modify the logDonation server action to check if the passed-in user ID is already a supporter for the non-profit. If it's not, add the user ID as a supporter.