Fix for mixes 34, 35/41, 40 (RTC-21499)

Export of offer items does not fail after the first offer group is removed from an offer. This applies to mixes in which we can add and remove offer groups: 34, 35/41, 40.

Combine from groups and get discount per group (RTC-21896)

Name and description of mix type 35/41 is changed to 'Combine from groups and get discount per group'. Discount can be specified for all groups in the offer with this mix type.