[MM-47566] Add upcominginvoice field to Subscription type (#21717)

* Add upcominginvoice field to Subscription type

* Add support for fetching upcoming invoices

* Put back upcominginvoice

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

web/context.go

@@ -743,7 +743,7 @@ func (c *Context) RequireInvoiceId() *Context {
 		return c
 	}
 
-	if len(c.Params.InvoiceId) != 27 {
+	if len(c.Params.InvoiceId) != 27 && c.Params.InvoiceId != model.UpcomingInvoice {
 		c.SetInvalidURLParam("invoice_id")
 	}