Turn on Threads for all users (out of labs)

[novocaine]

Make the threads feature available for all users.

Depends on #21351

[janogarcia]

@gsouquet It seems you accidentally modified the issue title.

[janogarcia]

@novocaine After internal discussion, it was decided not to provide a user setting to disable threads (it’s not entirely replacing an existing feature, it can lead to degraded experience for everyone as conversations get split between the thread and main timeline, etc.).

[germain-gg]

Using #21351 in place of this one