a hub's description page
  $db resolves                   <p>assembly $db</p>                -> <p>assembly hg38</p>
  ${db} in braces too            <p>assembly ${db}</p>              -> <p>assembly hg38</p>
  $$db stays a dollar            <p>$$db</p>                        -> <p>$db</p>
  $hgsid is NOT a variable       <img src="http://x/p?s=$hgsid">    -> <img src="http://x/p?s=$hgsid">
  ${hgsid} is NOT a variable     <img src="http://x/p?s=${hgsid}">  -> <img src="http://x/p?s=${hgsid}">
  a price is not a variable      <p>costs $5 million</p>            -> <p>costs $5 million</p>

a native description page
  ${hgsid} is not a variable     <a href="x?hgsid=${hgsid}">go</a>  -> <a href="x?hgsid=${hgsid}">go</a>
  $hgsid is not a variable       <a href="x?hgsid=$hgsid">go</a>    -> <a href="x?hgsid=$hgsid">go</a>
  $db was done by hgTrackDb      <p>assembly $db</p>                -> <p>assembly $db</p>
  a price is not a variable      <p>costs $5 million</p>            -> <p>costs $5 million</p>

nothing to substitute
  no dollar at all               <p>plain</p>                       -> <p>plain</p>
  empty page                                                        -> 
  a NULL track                   returned, no crash

