Add user types to profiles

Very simple change @houndci @cgruppioni @georgetmayorga
I could make them all strings and do an 
```
if profile.account_type = "Author"
```

For checks, but that would defeat the purpose of making the various account type classes as suggested by Steiner - basically, the idea was to make it so that if Authors can do different things that Bookshops, it would be best to make them a polymorphic class.

Unless I totally missed what you mean.