Adding SVGs for tramway lines T6 and T7 + white background for the M and RER logos

I added the SVG files for the yet open line T7 and the coming soon line T6. Didn't dare to touch the CSS files though, because my base64 outputs for unmodified SVGs were different from the ones in the CSS, so I felt insecure doing it.

Also, I added a white background for the metro and RER logos.