Danger symbol rework in SVG

[kocio-pl]

Trying to resolve one of the problems from #937.

The easiest thing would be to add 2 pixels h/w to danger.png. More desirable would be to recreate it in SVG.

I made a background to make it look the same, however the question is if we want this background or was it just a leftover we should get rid of:

Example at z13:

Before

After (with background)

After (without background)

[kocio-pl]

We can also change background opacity. For example with polygon-opacity: 0.5; it looks like:

Opacity 0.6:

Opacity 0.7:

[matthijsmelissen]

Perhaps we should decide on #684 and #2670 first?

[matthijsmelissen]

I'd prefer a wider solution that deals with all the military/danger_area rendering issues at once.

[kocio-pl]

Since these discussions are quite big and not active for at least a month, I think we could merge this as it is (making just PNG->SVG and 30px->32px change) to fix smaller issue. This would also allow to test new color ideas more easily than raster editing.