Set prevent enrollment pref checkbox to default checked

[jaredlockhart]

Ciprian raised a very good point which is that now that we can re enroll in rollouts, that could interact with the pref logic in desktop so that

1. A user enrolls in a rollout that targets prefs
2. The user manually changes the pref and is unenrolled
3. Nimbus syncs and then re-enrolls in the rollout, and if the feature writes to the user branch, could clobber the users settings

There's cases where we do or don't want that behaviour so it's not as clear cut as just always forcing the pref targeting check on for rollouts, but a potential mitigation is to default check it for rollouts and then provide a warning about this behaviour if it becomes unchecked.

┆Issue is synchronized with this Jira Task