CSS reset for svg max-width declarations

This PR is a fix for the issue described in #2881 

I think it makes sense to add it because the faulty CSS declaration that I mention there is by default included in every Joomla (popular opensource CMS) 3.x site and therefore effects millions of sites. It can't be fixed in the current Joomla version because of backwards compatibility considerations.
Can't we make this a bit less "catch all"? :)

Like `.leaflet-container .lealfet-map-pane img/svg` or something around ?