Svg scripts

* By using rounded rectangles, the resulting SVG is cleaner, and easier to extend
* The shield in the file name was redundant with the directory name
* The new file names allow for multi-line shields, and the scripts make allowances for these
* Using scripts makes it much easier to change all the shields at once for colour changes

These clear the way for me to work on #726 and https://github.com/gravitystorm/openstreetmap-carto/issues/670#issuecomment-48057334