Use chevron icon for local nav mobile menus

[adamwoodnz]

Closes #334 Depends on https://github.com/WordPress/wporg-mu-plugins/pull/480

Changes the local navigation blocks to use the 'menu' (3 bar) icon on mobile, so that in conjunction with https://github.com/WordPress/wporg-mu-plugins/pull/480 the button changes from the word 'Menu' to a chevron icon.

Screenshots

Page:State	Before	After
About:Closed
About:Open
Counter:Closed
Counter:Open

How to test the changes in this Pull Request:

1. Unsure your mu-plugins is on https://github.com/WordPress/wporg-mu-plugins/pull/480
2. Open a page from each About sub section, eg. requirements, domains, and philosophy, plus the download counter
3. Simulate mobile screen size
4. Check that the icon in the mobile menu button is a chevron
5. Open the menu
6. Check the the chevron icon has been flipper vertically
7. Close the menu

[jasmussen]

Change looks good!