Modify membership objects instead of replacing them in the MembershipList

[osks]

To reduce code complexity in controllers, we should not replace the membership objects or the unread_texts arrays, but rather keep the same objects and modify them when we need to update something. For example in the ReadConferenceTextsCtrl we have to $watch the membership to see if it changes so we always have the newest list of unread texts.

It will make the MembershipList code a bit more complex/hard, but it's just one place compared to multiple controllers (more so in the future).