Make announcement banner into a landmark (aside/complementary)

[gabalafou]

One of many fixes for #1428.

Axe-core wants: all page content should be contained by landmarks.

The announcement banner was not a landmark, nor contained by one, so this PR:

• changes the announcement from a div to an aside, making it into a "complementary" landmark
• adds the translatable string "Announcement" and sets it as the accessible label for the aside

This follows the implementation of the Black Lives Matter banner on a11yproject.com (i.e, an aside + aria-label).

I thought about putting both banners (version warning + announcement) together into a single landmark, but my thinking is that for sighted users, both banners are visually distinguishable and draw attention to themselves on every page, so I think making them both into separate landmarks achieves the same level of emphasis for both sighted and non-sighted users. It's also the simpler, more straightforward implementation.

After this PR, the landmarks read:

• Version warning complementary
• Announcement complementary
• banner
• navigation
• Section Navigation navigation
• main
• Breadcrumb navigation
• article
• footer

Note: we need to fix the section navigation landmark since it repeats the word navigation twice.

[trallard]

Thanks @gabalafou, this looks good!

Note: we need to fix the section navigation landmark since it repeats the word navigation twice. Assume this will be done in a separate PR?