migrate user activities

[santiaago]

This migration will happen in 3 steps:

step 1:

• Create a task queue that queries all users using a limit of users to process per task queue.
• For each user create an entity UserActivities that has a UserId and the list of ActivityIds of this user.

step 2:

Change the code of the app that uses User.ActivityIds to use UserActivities.Ids.

step 3:

• Create another task queue to remove the User.ActivityIds property and remove it from the User model.

Check how do I remove a field from a model in google app engine with go