Community validation

This allows users to validate a certificate.

It's currently deployed to staging, if you __sign in__ and visit [a certificate](http://staging.certificates.theodi.org/datasets/26/certificates/145) - then you will see a box to mark the certificate as valid:

![image](https://f.cloud.github.com/assets/51385/1454742/0ef2a290-430d-11e3-9d68-e58a066e9193.png)

Once you have marked it as valid, the ui will update to look like this:

![image](https://f.cloud.github.com/assets/51385/1454737/0065f7b8-430d-11e3-9b7a-8122e901402d.png)

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This UI can be totally changed.

Using it - I feel like it's missing a description or help text.