css: remove dividers between contacts

css: remove contact list margins (compensate with contact padding)

css: animate contact switching

index: expand, but also deduplicate existence checks for previous and contact

index: debug error stanzas in console

index: ignore messages of type 'error'

index: default message type is 'normal'

index: bjid variable is already defined, use it

messages: have a default Message stamp ("now"), use it as a shortcut Messages collection already has a comparator(), _splittable() and _foldable() that all depend on a defined stamp.

index: also cancel any pending subscription requests for removed contacts