Create update endpoints

We need to have the ability to modify certain pieces of information after they have been created.

[ ] Create UPDATE endpoint for changing user information (such as password or preferences (which haven't been implemented yet))
[ ] Create UPDATE endpoint for changing an activity's information
[ ] Create UPDATE endpoint for changing a date's information (primarily it's time)
[ ] Create UPDATE endpoint for updating multiple activities at once

BYUCS452F2021 / Plan-Bae