This defers the work to request the IM token when displaying a widget until
after we know that widget will actually be shown and the user has granted
permission for this widget to be displayed.
This is useful in general to avoid making unnecessary token checks, but it's
particularly helpful with the new Terms API, so that we only show the Terms flow
when widget are actually being used (as opposed to entering the room where
widgets exist, but haven't been shown / permitted).
Part of https://github.com/vector-im/riot-web/issues/10088
09c2d899ac