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