Add Identity Card Number to User

Fixes https://github.com/mysociety/alaveteli/issues/1700

Adds the ~~`id_number`~~ `identity_card_number` attribute and makes it available for the user to edit and see.

~~Need to add censor rules and include it in the initial request email.~~

Example of redaction:

![screen shot 2014-09-03 at 12 29 40](https://cloud.githubusercontent.com/assets/282788/4133382/c104ed44-335d-11e4-9909-9ad40a152810.png)
Ah I either misread the docs and need `.where(:text => identity_card_number).first_or_create`, or this got accidentally deleted through a rebase. Wonder why the spec passes…