User creation should also create a user slug

[julianlam]

A user account is currently accessed via URL by their username (/users/julian), although for some social accounts (namely Facebook and Google), spaces are used, which while not technically broken, could do with some prettifying, since it currently saves as (/users/Julian%20H.%20Lam).

On user creation, a slug should be assigned to the user, and this should be the user's canonical username (as opposed to a display name)