Channels and groups should only commit on next

@xander-miller pointed out a discrepancy in our new onboarding

When you tap on a channel or group, you are subscribed or unsubscribed immediately, which makes the skip button a little misleading.

We can fix this by:

Saving a dirty state of selected groups and channels and then committing those on next Remove the skip button (since next just moves them to next screen) and their choices are already committed.

We'll need to do the same thing on web.