Only make column headers bold, not row headers

Having row headers is very important for accessibility.
But all of our table headers are bold,
and that often doesn't look good with long row headers.
People who understand table semantics would need to overwrite that.
And people who don't understand semantics might choose not to use them.

assets/sass/elements/_tables.scss

@@ -16,7 +16,7 @@ table {
     border-bottom: 1px solid $border-colour;
   }
 
-  th {
+  thead th {
     font-weight: 700;
   }