Rewrite admin_booking.js using htmx

[spookylukey]

Probably most of the functionality of admin_booking.js could be replaced using htmx patterns i.e. partial page updates.

How easy this will be will probably depend mostly on how easy it is to override the templates to add htmx hx- attributes. Because the base templates are provided by Django, the amount of overriding needed may cancel out any advantages compared to the current approach based on jQuery and adding enhancement in that way.

[spookylukey]

We could add hx attributes dynamically, but this might add more indirection than the current approach.