Update handling of workshops for rollover memberships

[maneesha]

When a membership is rolled over to a new membership, we have the option of carrying over unused benefits. The number of workshops that can be rolled over should correspond with the number of workshops of each type in the source membership.

For example, if a member site had 2 inperson and 1 online workshop remaining, they may roll over at most 2 inperson workshops and 1 online workshop.

[elichad]

Location: fiscal/views.py > MembershipCreateRollOver

1. Update get_initial and get_form_kwargs and form_valid to pass through the rolled over values for all changed fields. You can copy the lines that are there for the original fields.
2. Update the tests in fiscal/tests/test_membership.py > TestMembershipCreateRollOver