User-friendly handling of max concurrent WiFi users

[rgaudin]

Limit the number of WiFi clients to 23 and reserve the last slot to display a captive-portal-like message that explains the Hotspot has reached WiFi capacity (and disconnect).

See https://github.com/offspot/shop/issues/192

[benoit74]

Probably you already have this in mind, but I feel like it is important to have this limit configurable somewhere should client decide to use another Wifi adapter.

[rgaudin]

We don't support other WiFi devices at the moment but indeed to me this should be configurable (disabled is good in known groups)

I just opened the ticket as per @kelson42 suggestion but there has been zero investigation nor thinking yet. Deserves move thinking and discussion IMO.