Set modal header/title size to match design spec

Set line height of title to 24px as given in spec
Given 16px padding, total header height is now 56px as given in spec

Use variables for padding/margin for close button (bootstrap now >4.2)

app/assets/stylesheets/framework/modal.scss

@@ -19,14 +19,14 @@
     }
   }
 
-  // leave enough space for the close icon
   .modal-title {
+    line-height: $gl-line-height-24;
+
+    // leave enough space for the close icon
     &.mw-100,
     &.w-100 {
-      // after upgrading to Bootstrap 4.2 we can use $modal-header-padding-x here
-      // https://github.com/twbs/bootstrap/pull/26976
-      margin-right: -28px;
-      padding-right: 28px;
+      margin-right: -$modal-header-padding-x;
+      padding-right: $modal-header-padding-x;
     }
   }
 }