cubicweb-conference #736329 problem with sponsoring level [resolved]

sponsoring level is an attribute of a sponsor, but a sponsor can sponsor more than one conference. the problem is that the sponsoring lever of a sponsor is not necessarily the same for every conference it sponsors.

done in0.4.0
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closed by#6d75578c6a8b A sponsor can support some conferences with various levels of participation (closes #736329)