Add ability to filter by tags in kiwix serve

This change introduces filtering by tags.
To filter, the user can click on the tag name and it will filter it.
A label is added (clickable) to show the tag filter, it can be clicked to remove the filter

static/templates/index.html

@@ -58,6 +58,7 @@
       </div>
       <form id='kiwixSearchForm' class='kiwixNav__SearchForm'>
         <input type="text" name="q" placeholder="Search" id="searchFilter" class='kiwixSearch filter'>
+        <span class="kiwixButton tagFilterLabel"></span>
         <input type="submit" class="kiwixButton kiwixButtonHover" value="Search"/>
       </form>
     </div>