fix: Focus accessibility (#1536)

* fix: Focus ring appearing on click
fix: Focus ring not appearing on sidebar links
add: focus-visible polyfill

* fix: More visible outlines on keyboard focus
fix: Header block should be a button semantically

package.json

@@ -88,6 +88,7 @@
     "exports-loader": "^0.6.4",
     "file-loader": "^1.1.6",
     "flow-typed": "^2.6.2",
+    "focus-visible": "^5.1.0",
     "fs-extra": "^4.0.2",
     "google-auth-library": "^5.5.1",
     "http-errors": "1.4.0",