Reboot updates

[MartijnCuppens]

• Inherit box-sizing and set box-sizing: border-box; on body
  This makes it possible for 3th party components to use box-sizing: content-box by just applying the box-sizing to the wrapper div. Nope: Reboot updates #28917 (comment)
• Remove unsupported browsers fixes (IE10 & Firefox 39-)
• More consistent comment blocks
• Reorganise order of some blocks
• Add height fix for date/month/week/time inputs (Temporal inputs are 2px taller than other textual inputs in Chrome #18842, this was already fixed by adding a height to form controls, with this change we don't need to set the height anymore)
• Remove redundant type selectors for some pseudo elements
• Additional resets and theming options for legend
• Element theming from _code.scss moved to reboot
• All input fields now have the same borders

Todo:

• Merge code page with reboot page

Result: https://deploy-preview-28917--twbs-bootstrap.netlify.com/docs/4.3/content/reboot/

[mdo]

Regarding the box-sizing change: #22872

[mdo]

Last thing I've noticed here is that if we're moving all those styles to Reboot, we need to move the docs content to the Reboot page as well.

[MartijnCuppens]

I'm not sure what to do with the .pre-scrollable class. It's the only class left in _code.scss and I can't document it on the reboot page since it isn't part of it. Should we leave it as only class on the code page and in _code.scss or is it not worth it and can we drop it?

[ysds]

I agree to remove the .pre-scrollable. I'm OK remove it in this PR, but I prefer to separate for clarity.

[MartijnCuppens]

I've opened #29135 to drop the class.