Global Footer: Switch multiple nav blocks to lists inside a nav wrapper.

[ryelle]

Fixes #402 — This switches to the global footer block to use a nav group block around 5 lists, instead of 5 separate nav blocks. Additionally, the label "Footer" has been added to the nav, so it now appears as "Footer navigation" when browsing by landmark.

This also had to change the way menu items were localized, removing the filter on nav-link blocks in favor of directly calling the function on appropriate links.

Props @joedolson for the report

Screenshots

	Production	Branch
Home, wide screen
Home, small screen
Rosetta, wide screen
Rosetta, small screen
Classic theme, wide screen
Classic theme, small screen

To test

1. If you can, apply this to a sandbox and test on various sites— new & old themes, rosetta, etc.
2. Otherwise, use with one of the redesign sites (main, showcase, etc) to test locally
3. The footer is now one single nav element, individual items are lists
4. There should be no visual changes in the footer

Optionally test with a screen reader to see the single Footer navigation landmark.

[StevenDufresne]

Looks good. Noting that there's a non-consequential padding-right.

.wp-block-group.global-footer>*, .wp-block-group.global-header {
    padding-right: var(--wp--custom--alignment--scroll-bar-width);
}

[ryelle]

Good catch, that doesn't seem to be necessary since the footer items are wrapped in a group with side-padding. I've removed it in 3d3b06a.