Fix glitch in top row header in Grid component

This was not working in the case of a group with only one column (for
example "I/O" in the "Details for all databases" grid).

We rely on the rowspan value instead of colspan.

We also avoid duplicated code along the way.

powa/static/styles/main.scss

@@ -51,14 +51,13 @@ pre.sql {
   }
 }
 
-th[colspan]:not([colspan="1"]) {
-  border-left: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
-  border-bottom: 0 !important;
-}
-
-th[colspan]:not([colspan="1"]) {
-  border-left: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
-  border-bottom: 0 !important;
+tr:first-child th {
+  &:not([rowspan="2"]) {
+    border-left: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
+  }
+  &[rowspan="1"] {
+    border-bottom: 0 !important;
+  }
 }
 
 td {