[web] Unify behaviour of chat composer with native

Summary:
An updated version of https://phab.comm.dev/D5518 (in the new stack). Task: https://linear.app/comm/issue/ENG-4022/[web]-unify-behaviour-of-chat-composer-with-native

The web app should behave as the native app:

• when selecting someone in the chat composer who isn't a friend, and when nobody else is selected, the web app should exit the search and create a 1-1 thread with this user
• should display an alert if you try to add a non-friend when you have already selected some users
• should display an alert on blocked users

Test Plan:

• select a friend, create chat
• select a non-friend -> search should end and chat should be displayed without the search bar
• select a friend, than a non-friend -> alert should be displayed
• select a blocked user -> alert should be displayed

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8224