recent topics: Clarify label for filter box.

* We shouldn't use "Search" when we mean "Filter".  Filter is correct
  here, since we are just showing a subset of what's otherwise shown,
  and won't find anything that's older (or whatever).
* The stream/topic wording was unnecessary; the things we're filtering
  are topics (E.g. "Filter users" might look at name/email, and is the
  right label, not "Filter name/email").

static/templates/recent_topics_table.hbs

@@ -3,7 +3,7 @@
         {{> recent_topics_filters}}
     </div>
     <div class="search_group" role="group">
-        <input type="text" id="recent_topics_search" value="{{ search_val }}" placeholder="{{t 'Search stream / topic (t)' }}">
+        <input type="text" id="recent_topics_search" value="{{ search_val }}" placeholder="{{t 'Filter topics (t)' }}">
         <button type="button" class="btn clear_search_button" id="recent_topics_search_clear">
             <i class="fa fa-remove" aria-hidden="true"></i>
         </button>