Make the default time durations configurable on User Form Interface

[jonathanhle]

We'd like customize the selectable time durations--including having a different default duration--when creating an Access Request.

Updated README with new REACT_APP Environment Vars introduced:

**Environment Default Overrides:**

- `UNTIL_ID_TO_LABELS_OVERRIDE`: Allows overriding the default ID to labels mapping. **Example:** `'{"14400":"4 Hours","43200":"12 Hours","432000":"5 Days","1209600":"Two Weeks","2592000":"30 Days","7776000":"90 Days"}'`.
- `UNTIL_JUST_NUMERIC_ID_TO_LABELS_OVERRIDE`: Allows overriding the default numeric ID to labels mapping. **Example:** `'{"14400":"4 Hours","43200":"12 Hours","432000":"5 Days","1209600":"Two Weeks","2592000":"30 Days","7776000":"90 Days"}'`.
- `DEFAULT_ACCESS_TIME_OVERRIDE`: Allows overriding the default access time. **Example:** `43200` (in seconds).

Defaults for these overrides can be found in the `src/env-overrides.tsx` file.

Credit to @Jui1337 who worked on the initial logic for this during her Internship on our team, but didn't manage to get this upstream yet.

I believe we've respected the current settings by setting them as the defaults in src/env-overrides.tsx.

Let me know if you'd like any changes / if this can be accepted upstream.

[jonathanhle]

closing this PR in lieu of #212