Proximity of controls: comments form improvements

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Splitting this out from #43412.

In the comments form, the buttons to submit and cancel the form are placed at the opposite sides of the screen:

The larger the screen, the bigger the buttons distance:

As mentioned in the tracking ticket #40822, proximity of related information is an universal design principle which benefits everyone and it's particularly important for users with a reduced visual field, low vision, or other vision or cognitive impairments.

User interface controls that are logically grouped should be placed close each other. For references and related WCAG recommendations, please see #40822.

Additionally, the visual order and the DOM order mismatch and this is a specific WCAG violation. Worth also mentioning the primary action button should be on the left. See the detailed design feedback in https://core.trac.wordpress.org/ticket/40822#comment:9 and the following comment pointing out that's basically the pattern the new Block Editor (Gutenberg) already follows.

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45972.diff​ aligns the buttons on the left, for both the reply and quick edit forms. Example:

Note: the same change applies to the comments form in the Dashboard "Recent Comments" widget and in the Classic Editor comments meta box.

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45972.2.diff​ addresses a minor CSS issue.

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In 44757:

Accessibility: Improve buttons placement in the Comments Quick edit/Reply form.

Proximity of related information is an inclusive design principle that benefits everyone and it's particularly important for users with reduced visual field, low vision, or other vision or cognitive impairments. User interface controls that are logically grouped should be placed close to each other.
Also, this change makes the buttons visual order and the DOM order match.

Fixes #45972.