Improve row focus and keyboard nav in Item Getting Guide

Clearer focus states, row changes bg color on hover/focus to help you
track where you are, remove the redundant image thumbnail link from the
tab order (it's the same as the item name link!)

app/assets/stylesheets/items/sources.sass

@@ -30,7 +30,7 @@
 
 	.name-cell a
 		text-decoration: none
-		&:hover
+		&:hover, &:focus
 			text-decoration: underline
 
 	.actions-cell
@@ -38,6 +38,11 @@
 		padding-left: 1em
 		font-size: 85%
 
+	tbody
+		tr
+			&:hover, &:focus-within
+				background: rgba($module-bg-color, 0.5)
+
 	thead
 		background: $module-bg-color
 

app/views/items/_item_list_row.html.haml

@@ -1,4 +1,6 @@
 %tr
-	%td.thumbnail-cell= link_to item_thumbnail_for(item), item, target: "_blank"
+	%td.thumbnail-cell
+		= link_to item_thumbnail_for(item), item, target: "_blank",
+		  tabindex: "-1"
 	%td.name-cell= link_to item.name, item, target: "_blank"
 	%td.actions-cell= yield