Introduce drafts related types and redux actions

Summary: This differential defines types to store drafts in redux and actions to alter them via reducers. Note: setDraftStoreDraftsActionType si define temporarily to increase readability and enhance smooth code review process for this stack. It will be deleted shortly.

Test Plan: Make sure eslint and flow do not point any errors.

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, atul

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5643