Customer told us that they are used to a view of the booking status of
each campsite in the form of a grid: each campsite is a row, and each
day a column; bookings are show as boxes from the first day to the last
day on the corresponding campsite’s row.
I do not yet show the booking boxes, but at least now i have the grid
and date selector form in place.
In the form i would need a couple of input[type=month], but this is not
yet supported in Firefox and Safari. According to MDN, one common way
to bypass that problem is to have two fields, one for the month and the
other for the year; i just did that, but had to create a new input type
in the `form` package just for this.