Update volunteer availability to be any chunk of time.

[theosiemensrhodes]

Context

The way we keep track of availability is using a bitString where 0 is available and 1 is busy.

Issue

Currently volunteer availability only covers from 6am to 9pm every day. (See src/constants.ts)

We should just do 30 minute chunks for every individual time in the week.

Notes

• This issue will require a migration on the backend, and minor code changes. It does not cover frontend code changes.
• Make sure like previously the length of the bitString is validated before being inserted in the DB