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/settings/FontScalingPanel.tsx

@@ -128,6 +128,7 @@ export default class FontScalingPanel extends React.Component<IProps, IState> {
                         onChange={this.onFontSizeChanged}
                         displayFunc={(_) => ""}
                         disabled={this.state.useCustomFontSize}
+                        label={_t("Font size")}
                     />
                     <div className="mx_FontScalingPanel_fontSlider_largeText">Aa</div>
                 </div>