Add content IDs to contacts

This is in preparation for sending related contacts to the [content store](https://github.com/alphagov/content-store).

To support relationships between content items that still work when the target moves from one URL to another, we’ve decided to add a unique, immutable ID to each content item that’s only visible within the publishing system itself; the content store will translate these into URLs for published content.
I think you want `\z` here (lowercase).  From the [docs](http://apidock.com/ruby/Regexp)(way down in the 'Anchors' section): 

> `\Z` - Matches end of string. If string ends with a newline, it matches just before newline
> `\z` - Matches end of string