Show membership number in Identity

This fixes `MEM-159`.

Basically adds this 'Membership number' field to the Membership tab on Identity:

![image](https://cloud.githubusercontent.com/assets/394376/4703008/3a00f660-5867-11e4-87c0-5d8e75b9fbd5.png)


This field often resolves to `null`, either because:

a) the user is on a free tier and therefore doesn't have an ID in the system
b) the batch job which generates these numbers hasn't run yet (eg. the user signed up very recently)

This field only shows when the ID exists.
Fair point – done! I guess it's to allow other text in the element but we don't need that here.