Global header, Local navigation: Add aria label to the main, search, and local nav blocks

[ryelle]

Fixes #622 — This PR adds aria-labels to the 3 header nav elements. There are two in the global header, the main navigation element and the search toggle; and one in the local navigation bar just below the global header. This PR adds aria labels to each, so they are now "Main", "Search", and "Section", respectively.

<nav class="is-responsive global-header__navigation …" aria-label="Main" data-wp-interactive…

<nav class="is-responsive  is-vertical global-header__search …" aria-label="Search" data-wp-interactive…

<nav class="has-small-font-size is-responsive …" aria-label="Section" data-wp-interactive…

Note: There is a separate issue to merge the search toggle into the main navigation landmark #311, but that would require more refactoring of the header. For now, I've just added the aria label to clarify that nav too.

To test

• View a page with the global header (should be any page)
• The first two navigation elements have aria-labels
• View a page with a local navigation bar (About, a download subpage, one of the directories)
• The local navigation menu should also have an aria-label.

CC @bbertucc, @alexstine

[bbertucc]

Looks good to me.

[adamwoodnz]

Yep works for me, tested on Learn