Bug 26747: Show 'Always available' if title always available in overdrive

Bug 26747: Show 'Always available' if title always available in overdrive

To test:
1 - Have a developer account for overdrive, or library credentials
    https://developer.overdrive.com/
2 - Enter info into 'Overdrive' sysprefs and enable overdrive
3 - Search for 'house' in the opac
4 - Click the overdrive link
5 - Note some titles in the collection show 999999 copies available
6 - Apply patch
7 - Note they now say 'Always available'

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>