Settings: bring back Accessibility section

[pilgrim-brave]

Upstream Chromium has a short but useful Accessibility subpage (chrome://settings/accessibility) with links to OS-level captions preferences and a curated list of related extensions in the web store. We removed this section long ago, but we should bring it back.

[bsclifton]

We'll get a chance to revisit this when looking at Chromium 82. That will migrate from Polymer2 to Polymer3 and will require action on our side. See https://bugs.chromium.org/p/chromium/issues/detail?id=1026426 for more info