Store LinkedIn username

We need this in the DCP, in order to correctly allow linking to member user profiles.

lkrzyzan has raised that a LinkedIn OAuth login doesn't return the username, however with the r_basicprofile permission granted to the app, it's possible to request it with a second request.

Having authenticated against LinkedIn, make an authenticated request to http://api.linkedin.com/v1/people/~:(public-profile-url) which will return a result that looks like:

<person>
<public-profile-url>http://www.linkedin.com/in/pmuir</public-profile-url>
</person>

The portion of the URL after http(s)://(www.)linkedin.com/in/ is the LinkedIn username.

The request does need to be authenticated (the ~ refers to the currently logged in user). Full details at https://developer.linkedin.com/documents/authentication but a OAuth library should generate a correct URL.