Expand advice for optional icons with hidden labels

to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it
Split out/clarify ``<select>`` and ``multiple``
Callout for labelling of input groups

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)
Use aria-label for pagination prev/next

Cleaner alternative to using ``<span class="sr-only">``
Make main segmented button keyboard-focusable

unless there's a reason why ``tabindex="-1"`` was dropped in there...
grunt-saucelabs now uses `throttled` instead of `concurrency`

[skip validator]
Merge pull request #15327 from patrickhlauke/docs-accessibility-tweaks2

More docs accessibility tweaks
tweak browser bug list order so Safari bugs are contiguous
tweak browser bug list order so Chrome bugs are contiguous
Add grunt-contrib-compress.

Now, the zip file is created when running `grunt prep-release`.

[ci skip]
Update glob.
Merge pull request #15293 from twbs/grunt-contrib-compress

Add grunt-contrib-compress.
Tie help block to input with aria-describedby
Merge pull request #15330 from patrickhlauke/customizer-variables-help-block

Tie help block to input with aria-describedby
Fix/merge advice on low percentage progress bars

- actually add the ``min-width`` extra style (and use ``em`` rather than
``px``) to the 0% and 2% example
- remove the unnecessarily redundant sentence in the "With labels"
section
- remove the unnecessary heading/section specifically for low percentage
case, make it flow naturally as part of the "With labels" section
Filed http://openradar.appspot.com/19208483 for #12536
Add @patrickhlauke to Core Team list on the About page

[skip sauce]
[skip validator]
Merge pull request #15334 from patrickhlauke/progress-low-percentages

Fix/merge advice on low percentage progress bars
Merge pull request #15337 from twbs/patrickhlauke

Add @patrickhlauke to Core Team list on the About page
Specify `img` dimensions in the HTML.