Update to new version of slate
Lots of fixes here as a lot of the API has changed (eg. anchorKey /
offsetKey are now anchor.key and offset.key, and collapseFocusToThing
is moveFocusToThing).
Also changes the ref to a function (sorry for lumping this into the
same PR).
Hopefully will fix https://github.com/vector-im/riot-web/issues/7105
