Fix multiple accessibility defects identified by AXE (#10606)

* Mark effects overlay canvas as aria hidden

* Ensure date separators aren't seen as focusable aria separators

* Fix

* Fix font slider not having aria label

* Add missing aria labels

* Fix settings flags setting aria-checked={null}

* Update snapshots

src/components/views/elements/Slider.tsx

@@ -35,6 +35,8 @@ interface IProps {
 
     // Whether the slider is disabled
     disabled: boolean;
+
+    label: string;
 }
 
 const THUMB_SIZE = 2.4; // em
@@ -77,6 +79,7 @@ export default class Slider extends React.Component<IProps> {
                     disabled={this.props.disabled}
                     step={this.props.step}
                     autoComplete="off"
+                    aria-label={this.props.label}
                 />
                 {selection}
             </div>