Properly migrate rooms with a whitespace name

Those rooms need to get the default name and not a pure-whitespace string
Fix book/prebook button for admins
Fix room stats page
Fix location statistics page
Pre-booking for admins only when needed
Fix border-radius in i-button radios/checkboxes
Improve room stats page style and features

* Template style boost and clean up
* Displays pending bookings in the table
* Allows occupancy stats since ever
Add flexible days_between util function
Fix room stats calculations