Validate format of subdomains / schema names

Note we're using the `:HOST` regular expression since subdomains can
contain the same characters as domains do. This isn't 100% precise,
though, since subdomains have a maximum length of 63 characters, but is
good enough for our purposes.

spec/models/tenant_spec.rb

@@ -25,6 +25,16 @@ describe Tenant do
       end
     end
 
+    it "is valid with nested subdomains" do
+      tenant.schema = "multiple.sub.domains"
+      expect(tenant).to be_valid
+    end
+
+    it "is not valid with an invalid subdomain" do
+      tenant.schema = "my sub domain"
+      expect(tenant).not_to be_valid
+    end
+
     it "is not valid without a name" do
       tenant.name = ""
       expect(tenant).not_to be_valid