a11y improvements on primary navigation menu toggle

See #438. Also see https://core.trac.wordpress.org/ticket/27147.

Minor issue: before the change, there was no styling on the menu toggle. After the change, the menu toggle takes on the styles for buttons defined around https://github.com/Automattic/_s/blob/master/style.css#L256. 

![Primary Menu](https://i.cloudup.com/7E2E1LDtpE.png)

This is a matter of taste that I have no strong opinion about—removing all styles on the menu toggle or leaving the button styles intact. I also added the word "Primary" into the menu. This is also a matter of preference, but if it's accepted then we'll need to update the POT file for the project.