Support a wildcard/custom Salon-specific callback

[juniormonkey]

Occasionally we have a curator introduce a new callback just for this salon. Potentially we could reserve one callback button for the "wildcard", which (with a curator's forewarning!) could be configured to a once-off callback for individual salons? This would need:

• a wildcard callback button, with text and video/audio set at runtime
• configuration in the admin module to specify callback text and video/audio
• a heads-up from the curator, with a video/audio file to use for the callback