Flatten and simplify the memberlist sorting algorithm
The previous algorithm had a bug where it was getting stuck on the power level comparison, leaving the memberlist incorrectly ordered. The new algorithm uses less branching to try and walk through the different cases instead. Additionally, the steps used to determine the order have changed slightly to better represent an active member list.
This commit also includes changes to try and re-sort the member list more often during presence changes. Events are not always emitted, however. This may be a js-sdk bug but appears to happen prior to these changes as well.
Fixes https://github.com/vector-im/riot-web/issues/6953
