Add aria-expanded and aria-label to the sidebar section toggles.

[afercia]

This PR tries to improve the accessibility of the expandable section toggle buttons in the sidebar:

• adds an aria-expanded attribute
• adds an aria-label: sprintf( __( 'Open section: %s' ), title )

I'm intentionally avoiding to use the word "Toggle" because it's terribly difficult to translate in some languages, see core Trac ticket: https://core.trac.wordpress.org/ticket/34753

Also, one interesting consideration about the label: it's a bit controversial if the label text should be updated depending on the expanded/collapsed state. I've seen recommendations and examples for both ways. I think the best option depends on a case by case basis. In this specific case I'd say it makes perfectly sense to not change the label because aria-expanded already complements the label text; screen readers will announce the label + the state, for example:

Open section: Featured image, collapsed
Open section: Featured image, expanded
in this case, changing the label to
Close section: Featured image, expanded
could be even confusing.

Screenshots:

Note: safari displays the icons a bit misaligned, that's unrelated and already addressed in a separate issue see #1103

Fixes #1112

[ellatrix]

This is working great! 👍

[afercia]

@iseulde thanks for reviewing!