changing activity feed to fade in/out rather than slide up from the bottom of the screen

opentech/static_src/src/sass/apply/components/_activity-feed.scss

 .activity-feed {
     position: fixed;
-    top: 140px;
+    top: 0;
     right: 0;
     bottom: 0;
     left: 0;
     z-index: -1;
     opacity: 0;
-    transition: top, opacity, $transition;
+    transition: opacity, z-index, $transition;
 
     &.is-open {
-        top: 0;
         z-index: 20;
         width: 100%;
         height: 100vh;
         background: $color--white;
         opacity: 1;
-        transition: opacity, top, $transition;
     }
 
     &__header {