Add support for `follow_request` notifications

See pleroma#1559 (closed)

For 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 type being 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)).