Invert dark browse icons in dark mode

[AntonKhorev]

Some icons in the browse sidebar are too dark for dark mode. #4701 was a quick fix that could be applied to all icons. Here's another fix where only the selected dark icons are inverted.

In order to achieve that I did the following:

• icon styles are set with a Sass @each loop that goes over the icon data
• icon data is a nested Sass map where each icon has a filename and possibly an invert flag
• css properties are moved to common.scss to avoid Sass processing errors because they now use Bootstrap mixins and Bootstrap is included in common.scss

Node icons in dark mode:

Way icons are mostly unchanged.

Other required fixes:

• Tree icon needs to have white background removed. Actually a couple of other icons too, but they are inverted and white background is not noticeable. Fixed in Remove white background from tree, gate, level crossing icons #4736.

[tomhughes]

Very nice - all looks good to me, thanks.