MDL-57560 boost: File upload progress

Copy / adapt some styles from the bootstrap progress widget to match the DOM used for file upload progress.

Bootstrap changed this alot in the current beta so I opted to do a minimal change (affect the scss for boost only), rather
than adapt the HTML to something that will change again anyway when we upgrade to the release version of bootstrap 4.

theme/boost/scss/moodle/course.scss

@@ -544,13 +544,20 @@ span.editinstructions {
 }
 
 .dndupload-progress-outer {
-    @extend progress;
+    background-color: $progress-bg;
+    @include border-radius($progress-border-radius);
+    @include box-shadow($progress-box-shadow);
+    display: block;
+    width: 100%;
+    height: $spacer-y;
+    margin-bottom: $spacer-y;
 }
 
 .dndupload-progress-inner {
-    .progress {
-        @extend .progress-bar;
-    }
+    background-color: $progress-bar-color;
+    @include border-left-radius($progress-border-radius);
+    display: inline-block;
+    height: $spacer-y;
 }
 
 .dndupload-hidden {