templates/content-index.tpl.html: the `TT²` algorithm can be simplified
because the new query returns less. We get the date of the show
before the free slot, and can increment it differently depending on
whether the previous show is on a Friday or not - catering for
weekends in other words!
templates/queries-index-mysql.tpl.html,
templates/queries-index-sqlite.tpl.html: using a 'find unused index
numbers' query available in many places on the Internet, but
modified to return the id number and date for the show *before* the
next free slot.
Dave Morriss