Add admin.py for instructor_task.

This is primarily so that we can manually fail zombie instructor
tasks that are clogging up the queue for a course.

lms/djangoapps/instructor_task/admin.py

+"""
+Basic admin screens to search and edit InstructorTasks.
+
+This will mostly involve searching by course_id or task_id and manually failing
+a task.
+
+"""
+from django.contrib import admin
+from .models import InstructorTask
+
+class InstructorTaskAdmin(admin.ModelAdmin):
+    list_display = [
+        'task_id',
+        'task_type',
+        'course_id',
+        'username',
+        'email',
+        'created',
+        'updated',
+    ]
+    list_filter = ['task_type', 'task_state']
+    search_fields = [
+        'task_id', 'course_id', 'requester__email', 'requester__username'
+    ]
+    raw_id_fields = ['requester']  # avoid trying to make a select dropdown
+
+    def email(self, task):
+        return task.requester.email
+    email.admin_order_field = 'requester__email'
+
+    def username(self, task):
+        return task.requester.username
+    email.admin_order_field = 'requester__username'
+
+admin.site.register(InstructorTask, InstructorTaskAdmin)