invites: add "User Label" 1/2

Adds a "User Label" invite field, which is a label applied to users
created with it. This commit contains everything apart from the code to
apply it on account creation.

storage.go

@@ -500,6 +500,7 @@ type Invite struct {
 	Notify                     map[string]map[string]bool `json:"notify"`
 	Profile                    string                     `json:"profile"`
 	Label                      string                     `json:"label,omitempty"`
+	UserLabel                  string                     `json:"user_label,omitempty" example:"Friend"` // Label to apply to users created w/ this invite.
 	Captchas                   map[string]Captcha         // Map of Captcha IDs to images & answers
 	IsReferral                 bool                       `json:"is_referral" badgerhold:"index"`
 	ReferrerJellyfinID         string                     `json:"referrer_id"`