Add inbox follow request handling

- Not tested yet
- Undo needs to be extended to also handle undo of follow

storage-new/models/UserToUserRelation.go

@@ -9,5 +9,5 @@ type UserToUserRelation struct {
 	UserId       string
 	TargetUser   User // The user Y described in [RelationType]
 	TargetUserId string
-	Relation     RelationType `gorm:"type:relation_type"`
+	Relation     string `gorm:"type:relation_type"`
 }