Fix focus outline on budget executions index

The outline was invisible when we had the link containing block
elements, and I didn't manage to fix it, so the easiest solution is to
use an inline link and style the card with CSS.

app/assets/stylesheets/participation.scss

@@ -1284,6 +1284,7 @@
 }
 
 .budget-execution {
+  @include card;
   border: 1px solid $border;
   overflow: hidden;
   position: relative;