Reduce user last seen update frequency

We currently update the last seen value every 5 minutes, but this causes
the whole user row to be duplicated - bloating indices and the table.
We only use the value of last seen as a last login time, really, and so
do not need 5 minute granularity.

src/main/java/uk/ac/cam/cl/dtg/segue/api/Constants.java

@@ -557,7 +557,7 @@ public static SchoolInfoStatus get(final boolean schoolIdProvided, final boolean
     public static final int NEVER_CACHE_WITHOUT_ETAG_CHECK = 0;
 
     public static final String ANONYMOUS_USER = "ANONYMOUS_USER";
-    public static final int LAST_SEEN_UPDATE_FREQUENCY_MINUTES = 5;
+    public static final int LAST_SEEN_UPDATE_FREQUENCY_MINUTES = 15;
 
     /**
      * Redirect response field name.