recent_topics: Fix search box not visible after wrapping in safari.

On safari, after search box wraps to next line on smaller widths,
it is not visible due to some flex box default property difference
between chrome and safari. We fix this by resetting default
property.

static/styles/recent_topics.css

@@ -77,6 +77,8 @@
         #recent_topics_filter_buttons {
             margin: 0 10px 0 10px;
             display: flex;
+            /* Search box has no height without this in safari. */
+            flex: 0 0 auto;
             flex-wrap: wrap;
             justify-content: flex-start;
         }