Add notification privacy option to user settings

Exposes BE functionality we've had for a long time now

Correctly sets it in the database:

notification_settings     | {"follows": true, "followers": true, "non_follows": true, "non_followers": true, "privacy_option": false}

mentioned in issue pleroma#1745 (closed)

merged

mentioned in commit 28938340

changed milestone to %2.0.4

changed milestone to %2.1