After joining a community, we check if a link is a thread link and join a thread if necessary.

https://linear.app/comm/issue/ENG-8250/join-a-thread-after-joining-a-community

Depends on D12424

Tested following cases:

1. Accepting a community link when a user isn't a member of the community
2. Accepting a thread link when a user isn't a member of the community
3. Accepting a thread link when a user is a member of the community and isn't a member of the thread
4. Clicking a community link when a user is a member of the community
5. Clicking a thread link when a user is a member of the thread
6. Clicking an invalid link
7. Accepting a thread link when a user isn't a member of the community with a modification on the server side so that joining a thread fails. This results in joining the community and a modal with This invite link may be expired. Please try again with another invite link. message.