add secondary domains

note that for updates to work, we keep the secondary domain optional,
even though they are really not.

part of #809

migrations/schema.sql

@@ -218,7 +218,8 @@ CREATE TABLE IF NOT EXISTS subdomains(
     appId VARCHAR(128) NOT NULL,
     domain VARCHAR(128) NOT NULL,
     subdomain VARCHAR(128) NOT NULL,
-    type VARCHAR(128) NOT NULL, /* primary or redirect */
+    type VARCHAR(128) NOT NULL, /* primary, secondary, redirect, alias */
+    environmentVariable VARCHAR(128), /* only set for secondary */
 
     certificateJson MEDIUMTEXT,