Explorer: Add "About" section. Make internal code simplifications. (#…

…1997)

* Explorer: Adjust for validator.w3.org

Small changes, mainly to pass validation.

* Explorer: Tone down whites in dark mode

* Explorer: Change some table header labels

* Explorer: Keep table headers centered

* Explorer: Remove MakeCellEx and pass eslint.org

* Explorer: Code formatting

- Reformat with prettier.io.
- Replace tabs with two spaces.
- Use single quotes as default.
- Reduce code length.

* Explorer: Correct a color in dark mode

* Explorer: Add 'About' section

* Explorer: Add shadows on hover in menu

* Explorer: Display Genesis block if no height is given

* Explorer: Replace collapsibles with details/summary tags

Simplifies code (and no JS needed)

* Explorer: Add collapsibles to historical blocks

* Explorer: Adjust margins