Updated device types to collapse android mobile/tablet (bug 978150)

This is ready for review. There's quite a bit of refactoring renaming 'device_types' to 'platforms' but I think that helps us talk about what these actually are. The next patch coming is to add 'form_factors' to apps and the submit flow.

One intention here is to allow this to be reverted, so I'm not removing the device type data from the database yet. My hope is we won't need to do this.

![gma2a8v25qbyjeajsdt3izseiwfqwwwxlwjrzgi8edi](https://f.cloud.github.com/assets/1106/2412366/fada4a88-aad1-11e3-8ff5-6e445823b9b4.png)

![fq0o4_-vz7izg1mpl5kw6gbgoqt5fxkvp7egqjizzlq](https://f.cloud.github.com/assets/1106/2412369/02aded28-aad2-11e3-836f-363d6df77e03.png)