Add basic SDG Management content section

Note using `params[:relatable_type].classify` is recognized as a
security risk by some tools. However, it's a false positive, since we've
added constraints to the URL so that paramenter can only have the values
we trust.

app/assets/stylesheets/admin/menu.scss

@@ -36,7 +36,8 @@
     }
 
     &.budgets-link,
-    &.investments-link {
+    &.investments-link,
+    &.budget-investments-link {
       @include icon(chart-pie, solid);
     }
 
@@ -56,7 +57,8 @@
       @include icon(envelope, regular);
     }
 
-    &.legislation-link {
+    &.legislation-link,
+    &.legislation-processes-link {
       @include icon(file-alt, solid);
     }