Accessibility: Change the "Show / Hide dismissed updates" link to a button.

For better accessibility and semantics, user interface controls that perform an
action should be buttons. Links should exclusively be used for navigation.
Also, adds an `aria-expanded` attribute to communicate the expandable panel state
and improves the buttons spacing.

Props Cheffheid, audrasjb, afercia.
Fixes #38674.


git-svn-id: https://develop.svn.wordpress.org/trunk@42785 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/common.css

@@ -1548,6 +1548,15 @@ ul#dismissed-updates {
 	display: none;
 }
 
+#dismissed-updates li > p {
+	margin-top: 0;
+}
+
+#dismiss,
+#undismiss {
+	margin-left: 0.5em;
+}
+
 form.upgrade {
 	margin-top: 8px;
 }