add link to PDB ligand in compound card

fix #218

ippisite/ippidb/templates/compound_db_links.html

-{%if compound.pubchem_id or compound.chembl_id or compound.chemspider_id %}
+{%if compound.pubchem_id or compound.chembl_id or compound.chemspider_id or compound.ligand_id %}
 <table class="table mb-0">
   <tbody>
     <tr class="row">
-      <td scope="col" class="col-4">
+      <td scope="col" class="col-3">
         {% if compound.pubchem_id %}
         <a href="https://pubchem.ncbi.nlm.nih.gov/compound/{{ compound.pubchem_id }}" target="_blank"><img src="/static/images/Other/Pubchemlogo.png" style="height: 1.5em;" title="PubChem ID: {{ compound.pubchem_id }}" /></a>
         {% endif %}
       </td>
-      <td scope="col" class="col-4">
+      <td scope="col" class="col-3">
         {% if compound.chembl_id %}
         <a href="https://www.ebi.ac.uk/chembldb/compound/inspect/{{ compound.chembl_id }}" target="_blank"><img src="/static/images/Other/Chembl_logo.png" style="height: 1.5em;" title="ChEMBL ID: {{ compound.pubchem_id }}" /></a>
         {% endif %}
       </td>
-      <td scope="col" class="col-4">
+      <td scope="col" class="col-3">
         {% if compound.chemspider_id %}
         <a href="http://www.chemspider.com/Chemical-Structure.{{ compound.chemspider_id}}.html" target="_blank"><img src="/static/images/Other/ChemSpider_Logo.png" style="height: 1.5em;" title="ChemSpider ID: {{ compound.chemspider_id }}" /></a>
         {% endif %}
       </td>
+      <td scope="col" class="col-3">
+        {% if compound.ligand_id %}
+        <a href="https://www.rcsb.org/ligand/{{ compound.ligand_id}}" target="_blank">{{compound.ligand_id}}</a>
+        {% endif %}
+      </td>
     </tr>
   </tbody>
 </table>

ippisite/ippidb/templates/compound_l_item.html

@@ -27,7 +27,7 @@
       <li class="list-group-item">InChiKey: {{ compound.inchikey }}</li>
       {% endif %}
     </ul>
-    {%if compound.pubchem_id or compound.chembl_id or compound.chemspider_id %}
+    {%if compound.pubchem_id or compound.chembl_id or compound.chemspider_id or compound.ligand_id %}
     <h4 class="pt-2 compound_list_title">External links</h4>
     {% include "compound_db_links.html" %}
     {% endif %}