Unit tests: Only display fixture on-screen for iOS

Handle multiple zero-offset Scrollspy elements.

When the first two elements in a scrollspy content block have a document
offset of zero (i.e. they're hard against the top of the page),
Scrollspy would switch between them on every scroll event.

This could happen, for example, in a system of nested sections:

```
<section id="animals">
  <section id="dogs">
	Content
  </section>
</section>
```

This ocurred because Scrollspy's check to see if it's at the end of the
array of sections uses `!arr[index]`. This misses the case where
`arr[index]` does exist and is zero.

This commit explicitly checks the array bounds.

[Docs / README] Fix URL to match the plugin keyword recommendation

[Docs] Replace accidently removed tooltip / popover text

Fix URL to match the keyword

X-Ref #15742

cc: @mdo

Since only on iOS tests fail when the fixture is displayed off-screen, see https://travis-ci.org/twbs/bootstrap/jobs/52605742

Use different way to make failing test more visible in Sauce

[Fixes #15928] Encourage use of icons with form validation states for co...

[Fixes #15928] Encourage use of icons with form validation states for colorblind users

Update text per @patrickhlauke's feedback

Update text per @patrickhlauke's feedback.

Fix stupid grammar mistake

[Fixes #15825] .initialism class should use mixin

Update all collapse triggers with .collapsed class and aria-expanded

Fixes #15741
Closes #15751 by merging it.

Unit tests: Use new names of module hooks

Refs http://qunitjs.com/upgrade-guide-2.x/#replace-qunit-jsdump-with-qunit-dump

Refs http://qunitjs.com/upgrade-guide-2.x/#replace-qunit-log-callback-with-qunit-log-callback-for-all-reporting-callbacks

Refs #15893

Modal: Apply any preexisting body padding again after closing

Closes #15291 by merging it.

Fixed performance bug where a tooltip element is created and then immediately destroyed when tooltip.hide() is called and there is no existing tooltip element

Resolves #15557 by merging it.

Fixes #15147.
Closes #15520 by merging it.

[Ref #15881] Use Explicit JS Variable Declarations rather than Chained

tip method in popover.js can be removed

[Ref #15881] Use Explicit Values for javascript variables rather than chained ones.

From https://github.com/twbs/bootstrap/pull/15881#discussion_r25330647

Add source code documentation of justified navbar for #15906