[lib] Block user from writing in sidebars

Summary:
ENG-38
This diff fixes the issue where blocked user could still write in the previously created sidebars.

Test Plan:
Tested between an emulator and web:

1. Create a personal 1-1 chat with other user, block the other user -> neither user can write nor edit properties of the chat and it's sidebars
2. Create a personal 1-1 chat with admin, block admin -> both user can still write and edit
3. Create a chat with 3 users, block one -> all users can still write
   • then remove the unblocked user -> neither user can write nor edit

Reviewers: tomek, atul, abosh, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5284