Steps to make tota11y more screen-reader friendly and keyboard-accessible

[jdan]

This PR adds several features to make tota11y more keyboard and screen-reader friendly, notably:

• Adding keyboard toggling to the list of plugins (and a focus indicator)
• Adding screen reader text to the toolbar expand button, close buttons in the info panel, and "scroll to error" buttons in the info panel entries
• When activating a plugin, focus is directed to the newly-created info panel
• Menu aria roles on the list of plugins

I am still struggling to make annotations screen reader friendly, as they are located near the errors to sighted users, but for screen readers there is no strong connection. I am thinking that the "Scroll to error" button can be repurposed to focus the violating element.

@rileyjshaw || @MichelleTodd - would you mind reviewing the code in this diff? a11y-wise, I'll keep exploring more options, but this is definitely a step in the right direction.

[rileyjshaw]

(Sorry that I haven't gotten to this yet - I can check it out by the end of the week if that's okay?)

[jdan]

Yeah no huge rush here. It's a big scary diff but there isn't that much going on. Mainly moving handlebars -> JSX (maybe that should be its own diff)

[allofthenorthwood]

all of this looks good to me! thank you for splitting your code up into such small & well-defined commits! :D

[rileyjshaw]

Woohooooooo jsx!!

Just took a peek at this, looks cool :)