Update contact list when contacts are deleted. #227

The logic in ConversationActivity for adding or updating items didn't allow for items being removed. We *could* come up with logic for doing that - throw all the items from the adapter into a set, remove any items corresponding to current contacts from the set, then batch update the adapter to remove any items remaining in the set - but I think it would be overly complex. It's simpler to just clear the adapter and add the current contacts.

Also tidied up some adapter code while looking for the bug.


See merge request !70