ConversationPicker shared checkmark asset with media editor toolbar. The
checkmark was a little bit too small, also checked and unchecked indicators
had different size.
This commit updates ConversationPicker to use the same checkmark styling
that group UI has.
In most cases when working with a ConversationItem we already have a
write transaction, but sometimes we don't. In this case, we know you
can only be blocked in a conversation if there's history in it (at
least as far as the local device knows), and so we can ignore any
threads that don't exist.
Creates a special data source enum for ConversationAvatarView data
providers. Before, ContactCell and ConversationAvatarView used the data
source, neither applying to each perfectly
