fix: make categories into accessible nav buttons

[nolanlawson]

fixes #219

1. Changes the categories <div> into a <nav> (see navigation role for details).
2. Changes each category button into a true <button>. This makes it tabbable, and you can press the space button to press them.
3. Adds aria-label for each button, since the title is not sufficient for a11y
4. Adds an aria-label to the <nav> to help distinguish it from any other <nav>s that may be on the page. This requires a new i18n label ("Emoji categories" by default).

Here is a GIF of me pressing tab and spacebar to demonstrate: https://gfycat.com/agedwindycockatiel

[nolanlawson]

Fixed a small CSS issue - I noticed that the user agent stylesheet for <button>s can override the fill now, so we can't use currentColor.