Adds advanced search SCSS module

** Why are these changes being introduced:

* The default rendering of the advanced search form is too compressed,
hampering legibility by users.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/post-114

** How does this address that need:

* This adds an SCSS module for the advanced search form, defining some
padding within form elements, and separation between the various panels
on the search form.
* We also cause the panel names to appear as bold text, to aid users
scanning down the form.

** Document any side effects to this change:

* None

Author: matt-bernhardt

asset/scss/content/_advanced-search.scss

@@ -0,0 +1,24 @@
+form#advanced-search {
+    input,
+    select {
+        padding: 1rem;
+    }
+
+    .advanced-search-content {
+        > div {
+            border-bottom: 1px solid $gray-l3;
+            padding: 2rem 0;
+        }
+
+        span.label,
+        div.field .field-meta label {
+            font-size: 2rem;
+            font-weight: bold;
+            padding-bottom: 1rem;
+        }
+    }
+
+    #page-actions {
+        padding-top: 2rem;
+    }
+}

asset/scss/content/_all.scss

@@ -1,3 +1,4 @@
+@import "advanced-search";
 @import "breadcrumbs";
 @import "navigation";
 @import "resource-list";