Use chevron icon for local nav mobile menu

[adamwoodnz]

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

Screenshots

State	Home	Single	Thanks
Closed
Open

Testing

1. Ensure your mu-plugins is on Add filter to replace local nav bar mobile menu icon wporg-mu-plugins#480
2. There are 3 different navs, so you need to test Home, Single, and Thanks pages
3. Simulate mobile size and check the menu icons when open and closed, a caret should be displayed for both

[jasmussen]

Looks good but the chevron needs to line up, currently it's offset to the right:

It should be more like this:

Nice work.

[adamwoodnz]

Looks good but the chevron needs to line up

Ah I forgot to update the screenshots after we updated it in mu-plugins

They're centrally aligned now, as you saw in the other repos 👍