rustdoc: clean up help and settings button CSS

The old version of this code specified a bunch of different numbers that had
to line up just right to get the size it wanted. This version uses flexbox
centering, specifies the font size, and lets the browser figure out the
rest of the layout automatically.

src/librustdoc/html/static/css/rustdoc.css

@@ -911,6 +911,7 @@ so that we can apply CSS-filters to change the arrow color in themes */
 
 .popover {
 	position: absolute;
+	top: 100%;
 	right: 0;
 	z-index: 2;
 	display: block;
@@ -1360,22 +1361,24 @@ a.test-arrow:hover {
 }
 #settings-menu, #help-button {
 	margin-left: 4px;
-	outline: none;
+	display: flex;
 }
 
 #settings-menu > a, #help-button > a, #copy-path {
 	width: 33px;
-	line-height: 1.5;
 }
 
 #settings-menu > a, #help-button > a {
-	padding: 5px;
-	height: 100%;
-	display: block;
+	display: flex;
+	align-items: center;
+	justify-content: center;
 	background-color: var(--button-background-color);
 	border: 1px solid var(--border-color);
 	border-radius: 2px;
 	color: var(--settings-button-color);
+	/* Rare exception to specifying font sizes in rem. Since this is acting
+	   as an icon, it's okay to specify their sizes in pixels. */
+	font-size: 20px;
 }
 
 #settings-menu > a:hover, #settings-menu > a:focus,
@@ -1411,14 +1414,6 @@ a.test-arrow:hover {
 	animation: rotating 2s linear infinite;
 }
 
-#help-button > a {
-	text-align: center;
-	/* Rare exception to specifying font sizes in rem. Since this is acting
-	   as an icon, it's okay to specify their sizes in pixels. */
-	font-size: 20px;
-	padding-top: 2px;
-}
-
 kbd {
 	display: inline-block;
 	padding: 3px 5px;