Admin "Select Shift Times" Calendar - Implementation

[sherryhli]

User Story and Context

As an admin (Sistering employee), I need to add the specific timeslots for shifts so volunteers can identify if they are available for the opportunity.

Acceptance Criteria

Product

Please see #42. Note that the attached design in #42 is outdated, here is a more recent version, however you should confirm with @amandatdu before proceeding.

This ticket covers the parts enclosed in the red rectangle only:

Figma link: https://www.figma.com/file/TWJTNWjK75A6VgSisySTLe/Admin-Experience?node-id=186%3A2199

Please confirm with @amandatdu on which designs to use.

Technical

• Create both a form component (in PostingShiftsCalendar.tsx under frontend/src/components/admin/posting)

Dev Notes

This ticket will require some investigation.

react-grid-layout:
https://www.npmjs.com/package/react-grid-layout