referrals: add "use expiry" option

adds an option when enabling referrals to use the duration of the source
invited (i.e., months, days, hours) for the referral invite. If enabled,
the user won't be able to make a new referral link after it expires. For
referrals enabled for new users via a profile, the clock starts ticking
as soon as the account is created.

models.go

@@ -424,7 +424,8 @@ type GetMyReferralRespDTO struct {
 	Code          string `json:"code"`
 	RemainingUses int    `json:"remaining_uses"`
 	NoLimit       bool   `json:"no_limit"`
-	Expiry        int64  `json:"expiry"` // Come back after this time to get a new referral
+	Expiry        int64  `json:"expiry"` // Come back after this time to get a new referral (if UseExpiry, a new one can't be made).
+	UseExpiry     bool   `json:"use_expiry"`
 }
 
 type EnableDisableReferralDTO struct {