small seat previews and additional strategies for preselected dates

[markus-96]

Hi, I added a checkbox that enables small previews in the zone. It shows the number of the seat and the username of the first Booking of the currently set timespan. When hovering over the seat, it still shows the bookings and assignments, but without the header.

The thoughts behind this feature is to have a possibility to quickly search on the map who is sitting where, without having to hover over every single seat.

Also, I added some strategies for the preselected dates that can be set via environment variables. You can set which day is preselected (today or tomorrow) and also if the current time of day should be selected as start or a predefined timespan. To make this work, there is also a toggle to turn off the restoration of the selected dates from the session storage.

And there was a bug in the Bookings Tab, there if you did not select both start and end dates, there would flash an error. It now checks if both (fromTS and toTS) values are set.

If you want to, I can also split it to seperate PRs.

*edit: added screenshot