Race condition in API

[Josh-Cena]

There are a lot of Sentry errors because someone could click the button twice in a row which causes DB write failure, or missing records, etc. Common examples:

• NO_FRIEND_REQUEST: caused by clicking accept friend req twice
• Unique constraint failed on the constraint: `netId_crn_season_worksheetNumber` : caused by adding the course twice (I think)

We should fix this on both backend and frontend:

• On backend, debounce all requests by (userID, payload)
• On frontend, change button into loading state until a response is received