This fixes a common React warning we see. Most of these components should be using constructors instead, however componentDidMount is just as good (and doesn't require converting most of these).
Conversion to classes will be done in a later stage of React warning fixes.
For https://github.com/vector-im/riot-web/issues/12877
Adds a UserView that contains a MainSplit with an empty div
and a RightPanel, preset to the given member.
UserView fetches the profile and creates a fake member, which
it passed on to the RightPanel.
this doesn't use the view_user action on purpose, to avoid any
interference of the UserView when trying to view a room member.
