Update theme form markup and styles

[creativecoder]

This change adds a handful of improvements to the markup and styles for the main plugin page, including

• Radio button labels and form field descriptions match other wp-admin page style and structure more closely
• All description text is tied to its input field with an aria-describedby attribute
• Theme tags have responsive column styles
• URLs are removed from translation strings

Testing instructions

• The changes should be markup structure and styles only
• Test theme creation at /wp-admin/themes.php?page=create-block-theme and make sure the form submits successfully

Before	After

[pbking]

They look like good changes, though I only did a cursory glance.

To be honest I would rather just completely remove the wp-admin interface rather than spend efforts improving it.

The only parts of the workflow that I know aren't available in the site editor are:

• export as a child theme/sibling theme
• include/manage screenshot

[vcanales]

I'm keen on getting this in, regardless of what we decide on for the future of the admin page; these changes are a valuable improvement for it in the meantime.

[creativecoder]

Thanks for the reviews and feedback! To me it seems worth merging this, even if we end up throwing it out soon in favor of other changes to the wp-admin section of the plugin.

Note that I added a couple more small tweaks based on feedback from running the form markup through validator.w3.org.

This seems ready to go, so I'll go ahead and merge it!

[creativecoder]

For anyone following here, I made a mistake in my final markup changes that removed a closing >. That's been corrected in a follow-up PR: #475