auth: add AuthUser.is_anonymous, along with some exposition

This reveals the name of the NotAnonymous decorator to be misleading,
an unfortunate detail only documented here, but which must be properly
resolved in a later changeset.

Note that NotAnonymous behaves as advertised as long as it is used
together with LoginRequired, which is always the case in the current
code, so there's no actual security issue here, the code is just weird,
hard to read and fragile.

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Some thoughts on cleaning this up in a future changeset: As it turns
out, every controller (except the login page!) should be LoginRequired
decorated (since it doesn't actually block anonymous users, as long as
anonymous access is enabled in the Kallithea config). Thus the most
obvious solution would be to move the LoginRequired functionality into
BaseController (with an override for LoginController), and delete the
decorator entirely. However, LoginRequired does one other thing: it
carries information about whether API access is enabled for individual
controller methods ("@LoginRequired(api_key=True)"), and also performs
the check for this, something which is not easily moved into the base
controller class, since the base controller doesn't know which method is
about to be called. Possibly that can be determined by poking Pylons,
but such code is likely to break with the upcoming TurboGears 2 move.
Thus such cleanup is probably better revisited after the switch to TG2.