Bug 32134: Show the bundle size when checked out

Bug 32134: Show the bundle size when checked out

When checking out a bundle, it can be useful to know the number of items
in this bundle, so that librarians can quickly check if items are
missing.

Test plan:
1. Create a bundle (see bug 28854 comment 458)
2. Check out the bundle.
3. Verify that you have the bundle size in the message below the barcode
   input ("Checked out: [...]. Bundle of X items. Due on [...]")
4. Checkout an item that is not a bundle.
5. Verify that the bundle message does not appear

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>