Fixes #18573: Change name of .form-control-label to .col-form-label (and .form-control-legend to .col-form-legend) to reiterate use for only grid layouts

Refs #16180, #16262
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Closes #19561
Closes #19567
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Closes #19339
Refs #19102, #19443, #19665
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Replace the leftover instances with .m-x-auto (and .d-block when necessary)
Closes #19665
Refs #19102
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Refs #19665
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Refs https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/5381673/
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Refs https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/
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Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1259972
Closes #19626
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https://crbug.com/595210

Closes #17338
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Refs #14839, #19088
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Closes #19532
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Sounds like IE11 isn't getting non-critical fixes anymore, so there's no point in mentioning it (just like there's no point in mentioning IE10).
Help us, Edge-Wan Kenobi! You're our only hope.
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http://bugzil.la/1139853 was resolved as a duplicate of http://bugzil.la/924068
Refs #15990
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The modal backdrop positioning bug related to iOS' virtual keyboard doesn't seem to reproduce in iOS 8.0+.
(Possibly as a side-effect of https://bugs.webkit.org/show_bug.cgi?id=153224 )
Refs #9023
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* https://bugs.chromium.org/p/chromium/issues/detail?id=273306
* https://bugs.webkit.org/show_bug.cgi?id=138192
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https://bugs.chromium.org/p/chromium/issues/detail?id=175502
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Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1217012
Refs #19393
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https://bugzilla.mozilla.org/show_bug.cgi?id=1253468
Closes #19390
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/fyi @mdo

Note: The selector in dashboard.css that used .page-header was superfluous
    because the margin that it zeroed was already zero even without that declaration.

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