vary dkim selector per mail domain

this is required for the case where the domain is added on multiple
cloudrons. initially, the plan was to just vary this as a derivation
of the dashboard domain. but this will break existing installation (wildcard
and manual domain setups cannot be re-programmed automatically).

migrations/schema.sql

@@ -174,6 +174,8 @@ CREATE TABLE IF NOT EXISTS mail(
     catchAllJson TEXT,
     relayJson TEXT,
 
+    dkimSelector VARCHAR(128) NOT NULL DEFAULT "cloudron",
+
     FOREIGN KEY(domain) REFERENCES domains(domain),
     PRIMARY KEY(domain))