Replace MapProxySource with some javascript code
Summary:
MapProxySource has only been added to QuickCharts master so is not
available in 5.70, so we cannot use it here. So replicate it using a bit
of JavaScript code.
Test Plan: Now works without 5.71
Reviewers: Plasma, ngraham, mart
Reviewed By: Plasma, ngraham, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29760