Example subdomain for wildcard-variant profiles

[stuartpb]

Post-#159/#160 (and, to a lesser extent, #29/#159), #122 is blocked (for wildcard profiles, at least) on an "example wildcard value" field to test if the profile (and its URLs) resolve(s). This would also be useful for reviewing profiles. ("demo" frequently works for this: it's what I found when assembling #165.)