[web] add starting payload to send reaction message dispatch action promise

Summary:
add starting payload to send reaction message dispatch action promise. This starting payload is the optimistically created reaction message.

Depends on D6120
Linear Task: ENG-2520

Test Plan: flow and this will be further tested in a subsequent diff

Reviewers: atul, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6121