[16.0][REF/ADD] Create membership_type, membership_custom_date

[carmenbianca]

membership_custom_date is a custom module used by Akretion/UGESS. It used to depend directly on membership_variable_period, which functionally made no sense, but was necessary because we re-used the membership_type field on product.template.

I've factored out that field into membership_type and made both membership_variable_period and membership_custom_date depend on that new module.

(Almost) no history was lost for membership_custom_date in copying the module over. 99% of the code was already in a single commit.