Draft: Button migration

What does this MR do?

This MR does two great big regex replaces to get all the remaining HAML buttons migrated to GitLab UI.

find	replace
(^(?!.*gl-button).*class:.*)btn (.*$)	$1gl-button btn $2
(^(?!.*gl-button).*)(\.btn[\.{])	$1.gl-button$2
btn-inverted	btn-secondary (where appropriate)

There were a couple of files that escaped the gaze of our regex queries, so I've manually updated those too. Because of that, I'm only 99% sure we've caught everything, but I'll take those odds.

Screenshots (strongly suggested)

😅

We'll want to go through these as a team in

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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