Code review request

[GoogleCodeExporter]

Branch name:
collections-backend

Link to the relevant commit(s):
https://code.google.com/p/oppia/source/detail?r=01a80959e2ad603fc686556f5eb28392
8fd8f0c3&name=collections-backend

Purpose of code changes on this branch:
To implement all necessary backend functionality to support collections in 
Oppia.

When reviewing my code changes, please focus on:
everything, especially:

1. Whether subscriptions are implemented correctly for collections
2. How well the rights management is done for collections
3. What sort of tests I might be able to do for rights management with 
collections
4. What we should do about the duplicated search functionality between 
explorations and collections
5. How well the collections tests cover the domain object and related services
6. Any other duplication between explorations and collections that you do not 
like

After the review, I'll merge this branch into: develop

Original issue reported on code.google.com by [email protected] on 21 Jul 2015 at 8:22