Send up user mission id too

Summary: We want to pass up the um id so that on task completion we can stick it onto the mission's list of completed tasks. You would think that we would already know where to put the task, but it gets confusing when tasks can belong to multiple missions and might be created without being tied to any mission at all! This change is betrothed to another in webapp.

Test Plan:
With both changes, open up world of math + 4th grade mission
In 4th grade, pick some common practice task
In devshell, do something like:
et = ExerciseTask.get_by_id(id)
et.user_mission.get()

to make sure that the et's associated mission is likely the math user mission

Complete the task in 4th grade
Refresh the page
Make sure you still see that task in the 4th grade mission's completed task list
(prior to this change, client-side it would plop it in the right list, but wouldn't persist there. The task would then appear in the world of math mission.)

Reviewers: benkomalo

Reviewed By: benkomalo

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5052