Add maskable icon, update colors

[raucao]

• Adds an icon with purpose "maskable" to the manifest. This ensures that PWAs installed from Chrome on Android get shaped by the vendor OS's icon specifications, same as native apps.
• Changes the topbar color to be a bit darker and uses the same background color for theme and loading background. When launching the PWA, this now shows the whole background as dark grey/blue, with the icon blending into that.

This is currently deployed to staging so you can test it.

[galfert]

Looks much better than the square icon :)

Also tested on iOS.