s

- Move 4.1 docs to 4.2
- Update versions everywhere to 4.1.3 with release script
- Manually bump the shorthand version in package.json
- Add 4.2 to the versions docs page
- Update some redirects
- Fix tests asset URLs
- Bump Nuget and more

This reverts commit af484dc1.

`shrink-to-fit=no` is not needed anymore - Apple removed the need for it from iOS9.3 onwards

See https://www.scottohara.me/blog/2018/12/11/shrink-to-fit.html and https://github.com/h5bp/html5-boilerplate/issues/2102

Having it uppercase doesn't offer anything and the lowercase version slightly improves gzip compression.

* use a trailing slash when possible
* use https when possible
* remove a few redirected links
* consistently use `https://popper.js.org/`
* fix `iconUrl` in nuget files
* change Jekyll Windows guide to the official one

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro

* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

[skip sauce]

Avoids cycling carousels when the page isn't visible.
Closes #17710
Refs #15566

faster and easier to test/develop js functionality not represented in unit tests, and gives us
a playground for interactions, etc.

It also makes it so developing javascript is now decoupled form jekyll, which should make everything
faster and less painful.

This commit also reverts my filter commit https://github.com/twbs/bootstrap/commit/9900771aa7f1b3ddcee49aec84082104776ace70
which broke scrollspy for dropdowns.