Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.

The actual em values were inconsistent with the docs,
while the docs were also inconsistent with the comments, namely:

* `sm` breakpoint was 34em which is 558px, not 480px.
  Changed to 480px, matching the docs and Bootstrap 3.
* `lg` max width was 60rem which is 960px, less gutter than `md`.
  Changed to 940px, same as Bootstrap 3.
* `xl` max width was 72.25rem which is 1140px not 1156px.
  Changed to 1140px matching the comment but not the docs.

All the values are now the same as they were in Bootstrap 3.
Addresses #17070 and #17388.

Adds table-responsive class to docs table

Remove the unused Google fonts.

Apply !default to SCSS variables

Missed one variable :(

Accept elements as the tooltip / popover content

When a DOM node is passed to an HTML tooltip, the `title` node is only
moved if it is not already in the tooltip. Otherwise, `empty()` is used
instead of `detach()` before appending the `title` to avoid memory
leaks. If a DOM node is passed to a plain text tooltip, its text is
copied via jQuery `.text()`.

Replaces `.detach()` with `.empty()`, as `.detach()` is almost never
useful but instead leaks memory. The difference between `empty` and
`detach` is that the latter keeps all the attached jQuery events/data.
However, since we do not return the previous children, the user would
have to keep these themselves, thus they can `detach()` if necessary.

This is a port of https://github.com/twbs/bootstrap/pull/14552 to v4.

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Use `https` when possible.

rm unused $state param from form-control-validation mixin

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Fix the JS errors due to jekyll-search.

Remove the now unused grunt-contrib-jade references.

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1) Update it to v1.0.8
2) Include jekyll-search where it's used only

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Update QUnit to v1.18.0.

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Remove defunct responsive-(in)visibility mixins

inline the one remaining usage into .hidden-print's definition
rm now-empty scss/_utilities-responsive.scss file

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Clear out non-v4 docs translations

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Bump year in jumbotron example

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Rebased & tweaked version of #17272

fixes #17097: Go back to using jQuery's text and html methods since innerText is nonstandard and not present in Firefox

Closes #17272 by merging a tweaked version of it.

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Add babel:dev to Sauce Grunt subtasks so we don't test stale JS