This experiments with making PageIssuesOverlay an Overlay with
various options.
The appending of children is a little messy and points at a need
to standardise this some way
(see https://phabricator.wikimedia.org/T209647)
TODO:
* Remove the iconString property on PageIssueSummary which is no longer
needed
Bug: T209647
Change-Id: Iadd798a820dca6bbb31edc9a8570b6db7aac237a
