Clean up about:torconnect styling

In preparation for #43570, and with other about:torconnect improvements in mind (#43185), I want to clean up some of the styling for about:torconnect:

1. Do not use error-pages.css. This made the previous page too quirky and meant we had to deal with styling interference from mozilla. We only need a few of our own CSS additions to cover the useful styling we inherited before.
2. Rather than share elements, each stage will get its own torconnect-stage-content element, which will show the corresponding stage. This should simplify the javascript once this is complete. And it should make future adjustments easier.
3. Change the body display to grid, which can dynamically handle the spacing of all elements. One improvement is that this will ensure that the progress bar's shadow will not overlap the title when the vertical height is narrow.
4. Simplify some of the styling spacing logic.
5. Some other minor clean ups.