Add groups index page

When render the investment list component with the link "see all
investments", now we redirect to groups index page when a budget has
multiple headings.

app/assets/stylesheets/budgets/groups/index.scss

@@ -0,0 +1,7 @@
+.budget-groups-index {
+
+  > header {
+    @extend %budget-header;
+    @include full-width-background($adjust-padding: true);
+  }
+}

app/assets/stylesheets/layout.scss

@@ -67,6 +67,7 @@ body {
 main {
   display: block;
 
+  &.budget-groups-index,
   &.budget-investment-new,
   &.debate-new,
   &.proposal-new,