PLT-3782 Do not move the "@" sign and flag icon to the left of search bar when opening the right hand sidebar (#4850)

* do not move the mention & flag button to the left of the search box when in search mode

* fixed the margin & padding issues of the search box

* fixed the member icon positioning issue and the expanded sidebar toggle issue

* fixed remaining bugs when sidebar is in expanded mode

* changed propTypes from func to bool

webapp/sass/responsive/_tablet.scss

@@ -20,6 +20,12 @@
         }
     }
 
+    .search__form {
+        .sidebar--right & {
+            width: 300px;
+        }
+    }
+
     .post-create__container {
         form {
             padding: .5em 0;