Rename 'Pending' column to something more descriptive

lms/static/coffee/src/staff_grading/staff_grading.coffee

@@ -344,7 +344,7 @@ class StaffGrading
         <tr>
             <th>Problem Name</th>
             <th>Graded</th>
-            <th>Pending</th>
+            <th>Available to Grade</th>
             <th>Required</th>
             <th>Progress</th>
         </tr>

lms/templates/peer_grading/peer_grading.html

@@ -31,7 +31,7 @@
                     <tr>
                         <th>Problem Name</th>
                         <th>Graded</th>
-                        <th>Pending</th>
+                        <th>Available</th>
                         <th>Required</th>
                         <th>Progress</th>
                     </tr>