[web] refactor validateState to operations

Summary:
This is similar to fixUnreadActiveThread on native.
Also moving processing ops to the end of reducer executions.

Depends on D9955

Test Plan:

1. Unread/read status works.
2. Processing ops works.

Reviewers: tomek, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9956