implement frontend for user expiry/duration

this will add an optional validity period to users, where their account
will be disabled (or deleted) a specified amount of time after they
created it.

storage.go

@@ -59,6 +59,10 @@ type Invite struct {
 	NoLimit       bool                       `json:"no-limit"`
 	RemainingUses int                        `json:"remaining-uses"`
 	ValidTill     time.Time                  `json:"valid_till"`
+	UserDuration  bool                       `json:"user-duration"`
+	UserDays      int                        `json:"user-days,omitempty"`
+	UserHours     int                        `json:"user-hours,omitempty"`
+	UserMinutes   int                        `json:"user-minutes,omitempty"`
 	Email         string                     `json:"email"`
 	UsedBy        [][]string                 `json:"used-by"`
 	Notify        map[string]map[string]bool `json:"notify"`