Add support for `follow_request` notifications
follow_request notification, "... requested to follow you" notification should be displayed.
(Would be nice to have in future versions: Approve/Reject button directly in notification, per pleroma#1559 (closed))
When user accepts notification,
/api/v1/notifications will render the same notification with
follow — please ensure it's not displayed as a new notification (ideally that should change previously displayed "... requested to follow you" notification to "... followed you" and mark it as read).
BE part (see description on enabling
follow_request notifications): pleroma!2354 (merged)
Related issue — Notifications panel crashes on unsupported notifications (#822 (closed)).