Bug 35475: Makes strings in booking JS file translatable

Bug 35475: Makes strings in booking JS file translatable

This makes several strings in the booking specific JS file
translatable.

To test:
* Make an item bookable:
  * Find a record with items in your catalog or create one
  * From the details page, switch to the items tab
  * Mark items as bookable
* Add a booking, verify the modal works as expected
* Check the bookings tab
* Verify the column headings of the bookings table show correctly
* Verify the "Biblio level" and "Item" in the calendar show
  Note: Months don't translate, this will be a separate bug
* Cancel a booking, edit a booking... make sure everything works
  as expected
* If you can: Install a translation and verify strings show up
  in po files as expected

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>