Add optimization for user_rel_tag table (index) taken from campus.chamilo.org

documentation/optimization.html

@@ -254,6 +254,7 @@ To solve this performance issue, you can execute the following queries manually
 <pre>
 ALTER TABLE lp_item ADD INDEX idx_c_lp_item_cid_lp_id (c_id, lp_id);
 ALTER TABLE lp_item_view ADD INDEX  idx_c_lp_item_view_cid_lp_view_id_lp_item_id (c_id, lp_view_id, lp_item_id);
+ALTER TABLE user_rel_tag ADD INDEX idx_user_rel_tag_user (user_id);
 </pre>
 In Chamilo 1.9.8, we use the c_item_property table more actively. This causes issues with the reporting pages for the assignments. You can reduce the impact by adding the following index:
 <pre>