warn when mapy.cz is used in a source tag

[Jakuje]

I went through the changesets and most of them use directly mapserver.mapy.cz (some older used mX.mapserver.mapy.cz so I hope this regex should work and would not cause any false positives.

See #9708

[tyrasd]

Adding this validation is fine, but note that it likely does not do what you might have intended with this PR: it only shows a warning when mapy.cz is used in a source tag of a map feature, (see screenshot below). It does not prevent users from using mapy.cz as a background layer.

[pnorman]

Given the original issue is solved, can this PR be closed?

[zbycz]

Yes, thanks. This can be closed. Solved via https://github.com/openstreetmap/openstreetmap-website/pull/4093