Fix hiding of completion/bookmark indicators in legacy courseware (#25919)

In the edx.org-next theme, the completion green check-
marks and the little bookmark icons were appearing on every
single unit. This was because a fontawesome CSS rule
in the new theme was overriding the `display: none`,
as set by `.is-hidden`, to be `display: inline: block`.

The fix is to add `!important` to a couple definitions
of `.is-hidden`. Definitely somewhat hacky, but this is
SCSS that we plan to deprecate at some point anyway.

TNL-7823

common/static/sass/_mixins.scss

@@ -388,7 +388,7 @@
 }
 
 .is-hidden {
-  display: none;
+  display: none !important;
 }
 
 // +Content - No List - Extends

lms/static/sass/base/_base.scss

@@ -251,7 +251,7 @@ mark {
 
 // UI - is hidden
 .is-hidden {
-  display: none;
+  display: none !important;
 }
 
 // UI - is deprecated