Import color variables in new page SCSS partials

"admin/all.css" which imports these SCSS partials already imports the
color variables in "admin/variables", so actually there should be no
need to import the variables again.

However, "application.css" calls "require_tree", which means asset
precompilation through Sprockets would attempt to compile each of the
SCSS partials individually. When compiled individually, the color
variables are not available to these partials.

This is a quick solution to allow precompilation of "application.css" to
complete.

app/assets/stylesheets/admin/pages/subscription_form.css.scss

@@ -1,3 +1,5 @@
+@import '../variables';
+
 .admin-subscription-form-subscription-line-items {
   .not-in-open-and-upcoming-order-cycles-warning {
     color: $warning-red;

app/assets/stylesheets/admin/pages/subscription_review.css.scss

@@ -1,3 +1,5 @@
+@import '../variables';
+
 .admin-subscription-review-subscription-line-items {
   .not-in-open-and-upcoming-order-cycles-warning {
     color: $warning-red;