expiry: add "remind N days before"

new setting to send an email/message N days before a user is due to
expire. Multiple can be set.

migrations.go

@@ -387,6 +387,9 @@ func intialiseCustomContent(app *appContext) {
 	if _, ok := app.storage.GetCustomContentKey("UserExpiryAdjusted"); !ok {
 		app.storage.SetCustomContentKey("UserExpiryAdjusted", emptyCC)
 	}
+	if _, ok := app.storage.GetCustomContentKey("ExpiryReminder"); !ok {
+		app.storage.SetCustomContentKey("ExpiryReminder", emptyCC)
+	}
 	if _, ok := app.storage.GetCustomContentKey("PostSignupCard"); !ok {
 		app.storage.SetCustomContentKey("PostSignupCard", emptyCC)