Reintroduce working resizer code for right panel
Fixes https://github.com/vector-im/riot-web/issues/11674
This re-introduces and adapted version of what was there before, but fixed for the new collapsed logic: https://github.com/matrix-org/matrix-react-sdk/pull/3703/files#diff-633a0248e235d7446a8868a9145efce2L77-L93
We no longer have a collapsedRhs variable and only set the panel when it is opened, so we can accurately track expanded/collapsed state through presence of a panel.
