Grab-bag of accessibility issues

[nolanlawson]

After merging #281 #282 #283 #284, there are still some remaining a11y issues I'd like to tackle:

• title emoji is decorative, should be aria-hidden
• title text should be a title for the whole thing, using aria-labelledby or something
• skintone picker should better communicate pressed and dropdown states
• the list of emoji in each category should use <ul>/<li>
• overall structure of regions/landmarks could be improved

[nolanlawson]

Given that VoiceOver seems to flatten all regions, we can probably remove the "list of emojis" section. That gives us one less i18n string to maintain anyway.

However, we should probably add "search" to the list of regions, since otherwise it's missing when you look at the list of regions.

[nolanlawson]

There could be additional improvements, especially in the skintone picker, but for now I think these are largely fixed.