hide empty brackets when not filtering through tags

lms/templates/discussion/_search_bar.html

@@ -9,10 +9,16 @@ def base_url_for_search():
 %>
 
 <form action="${base_url_for_search()}" method="get" class="discussion-search-form">
+  <%
+    if tags:
+      displayed_value = "[" + tags + "]" + text
+    else:
+      displayed_value = text
+  %>
   % if 'tag' in query_params:
-    <input class="search-input" type="text" value="[${tags}]${text}" id="keywords" autocomplete="off"/>
+    <input class="search-input" type="text" value="${displayed_value}" id="keywords" autocomplete="off"/>
   % else:
-    <input class="search-input" type="text" value="[${tags}]${text}" id="keywords" autocomplete="off"/>
+    <input class="search-input" type="text" value="${displayed_value}" id="keywords" autocomplete="off"/>
   % endif
   <div class="discussion-link discussion-search-link" href="javascript:void(0)">Search posts</div>
 </form>