Legacy URL Handler implementation

When dealing w very large numbers of legacy URLs that need to be redirected to the appropriate pages, sling:vanityPaths can reach limits.

This implementation is intended to be hooked into the 404 error page JSP .. ex impl integrated w the ACS EPH

```
<%@page session="false" import="com.adobe.acs.commons.legacyurls.LegacyURLHandler"%><%
LegacyURLHandler legacyURLHandler = sling.getService(LegacyURLHandler.class);

if(!legacyURLHandler.doRedirect(slingRequest, slingResponse)) { 
       // Returned false meaning no legacy URL match could be found
       %><%@include file="/apps/acs-commons/components/utilities/errorpagehandler/404.jsp" %><%
}
%>
```