Remove CSS extraction from stack and overview map

Refactor the overview map to defer to CSS to set the map container size
and compute overview scale from the computed style instead of parsing
CSS rules. Remove unused CSS extraction from the stack view. Since these
were the only dependents on rule order in `stack.css`, remove rule order
warning comments.

Progress towards removing `getPropertyFromCssRules`.
Remove CSS extraction from slider and cropping

Refactor slider widget to not depend on extracted CSS properties. The
slider handle is now positioned as a percentage of the slider, instead
of in absolute pixels. Slider and handle margins are now controlled by
CSS rather than through JS styling. Non-breaking spaces used as
positioning were removed from slider labels accordingly. CSS extraction
was also removed from sliders in the cropping tool. Since these were the
only dependents on rule order in `slider.css`, rule order warning
comments were removed.

Progress towards removing `getPropertyFromCssRules`.
Tools: remove CSS rule extraction

Remove `getPropertyFromCssRules` and helper function `getCssRules`
because they create brittle dependence on the inclusion order and rule
order of CSS files.