Timesheet API: implement functionality for submitting status changes

[izzyconner]

**Goal: We want to be able support the ability to see status changes as employees, supervisors, and admins take actions such as submitting & reviewing timesheets. **

IMPORTANT NOTE: This ticket should not be completed until this ticket is completed https://github.com/Code-4-Community/breaktime-backend/pull/31/files

**Schema: **
Attached to the uploadTimesheet endpoint there is a supported request body that allows for submitting status changes. We want to use that to be able to support 4 different status changes:

• "employee submitted their timesheet"
• "supervisor submitted their timesheet"
• "supervisor schedule submitted"
• "admin reviewed"

The uploadTimesheet endpoint should take in a body and the backend should be able to parse out what status change is necessary for which timesheet.

**Questions To Think About: **
We need to figure out what the resubmission policy is, when a submission is seen to be validated, and how to save changes to backend.