Bug 13726: Make Koha::Acq::Bookseller using Koha::Object

Bug 13726: Make Koha::Acq::Bookseller using Koha::Object

This patch create a Koha::Acquisition::Booksellers module and
Koha::Acquisition::Bookseller::Contract[s] modules.

All code in the acquisition module is adapted to use the CRUD methods of
Koha::Object[s].
The former C4 routines are removed.

Test plan:
Since a lot of files are impacted by this patch, try a complete
acquisition workflow and try to catch errors.
Be focused on bookseller and bookseller' contacts data.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>