Export screen: missing labels and accessibility improvements

[afercia]

In the Export screen, form fields do have labels but they're not correctly associated. We could also further improve semantics and accessibility grouping together form controls that are logically grouped, using fieldsets and legends.

Would like to discuss a bit with the accessibility team what should we do with form elements that are revealed inside the expandable panels: as soon as users select the Posts or Pages radio buttons, additional form controls get displayed but screen reader users don't get notified they've just got additional options. See screenshot below:

[afercia]

First pass:

• associate form labels with their controls
• use fieldset and legend elements to group related form controls
• use aria-describedby for the first radio button description
• improve a bit the layout on small screens

See screenshot (with the two panels expanded for demoing purposes):

[rianrietveld]

Accessibility user test results:
​https://make.wordpress.org/accessibility/2015/08/03/accessibility-usertest-export-xml-file/

[afercia]

Refreshed patch after the recent changes in [34326]. Fixes also the newly added labels association. Assigning to @wonderboymusic for his kind review and feedback.

[wonderboymusic]

In 35316:

Export: add missing labels and improvement accessibility of the Export screen.

Props afercia.
Fixes #33046.