[dist] Use free space in VG when calculating OBS_WORKER_ROOT_SIZE

If there is a preexisting logical volume in the OBS volume group the
calculated size for the worker root volumes is too large and the storage
setup fails. It is better to use the number of free physical extents in
this case.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
[webui] Use data: confirm: instead of :confirm for popups
[webui] Make project_bread_crumb compatible with engines
[webui] use named route for toggle_watch too