MDL-59870 users: Default align participant table header row.

Instead of putting vertical-align: middle as indication for participants table
headers, none is set which leads to theme default, i.e. bottom in Clean and Boost.

theme/bootstrapbase/less/moodle/user.less

@@ -128,9 +128,12 @@
 .userlist table#participants {
     text-align: center;
 }
-.userlist table#participants td,
-.userlist table#participants th {
+.userlist table#participants td {
+    text-align: left;
+    padding: 4px;
     vertical-align: middle;
+}
+.userlist table#participants th {
     text-align: left;
     padding: 4px;
 }

theme/bootstrapbase/style/moodle.css

@@ -9740,9 +9740,12 @@ body.path-question-type .mform fieldset.hidden {
 .userlist table#participants {
   text-align: center;
 }
-.userlist table#participants td,
-.userlist table#participants th {
+.userlist table#participants td {
+  text-align: left;
+  padding: 4px;
   vertical-align: middle;
+}
+.userlist table#participants th {
   text-align: left;
   padding: 4px;
 }