Minor UI improvements (#4959)

* PLT-5128 - Fixing channel header responsively

* PLT-4984 - Fixing search popover behaviour

webapp/sass/components/_popover.scss

@@ -93,20 +93,18 @@
 }
 
 .search-help-popover {
-    @include single-transition(opacity, .3s, ease-in);
     font-size: em(13px);
-    max-width: 350px;
-    top: 36px;
-    visibility: hidden;
+    margin-top: 17px !important;
+    max-width: 300px;
     width: 100%;
 
     &.autocomplete {
         display: block;
 
         .popover-content {
+            -webkit-overflow-scrolling: touch;
             padding: 10px;
             position: relative;
-            -webkit-overflow-scrolling: touch;
         }
     }
 
@@ -205,11 +203,6 @@
     .tooltip-inner {
         max-width: 100%;
     }
-
-    &.visible {
-        @include opacity(1);
-        visibility: visible;
-    }
 }
 
 .member-list__popover {