number of unknown speakers redone

[ceciliasharp]

Today we have a settings in backoffice for number of speakers, that is used to generate "unknown" speaker images on the speaker list. But that isn't completely true. If some speaker have two sessions, or if a session have two speakers that will not be correct. The setting in the back office should be number of sessions instead and we should count number of sessions expected - number of sessions from the speakers published.