Designate common positioning classes as `!important` a la Bootstrap

What does this MR do?

Set the common positioning classes as !important, like Bootstrap.

Consider the use case of SVGs in buttons: we have a global rule that sets top: 2px; on all .btn svgs.

In many cases, this 2px bump is unnecessary, and we have many cases where we now have to give icons classes (most commonly .icon), and then add CSS rules that resets top back to 0.

Perhaps as a intermediary step towards deciding whether that two pixel bump is required in the long term, one could simply use the prepend-top-0 class instead of icon, and not write any duplicative CSS!

What are the relevant issue numbers?

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• Changelog entry added, if necessary
• Documentation created/updated via this MR
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