Replace g.user with current_user proxy

[jace]

Flask-Login provides a current_user proxy that is available everywhere, including in templates.

Flask's documentation on local proxies hints at a crucial advantage: the backing function of a proxy is only called the first time the proxy is accessed.

Flask-Lastuser currently sets g.user on every request, even if the request needs no user authentication check. By switching to a proxy, the user can be authenticated only if required.

There will be no performance penalty with this switch, but:

1. All uses of g.user across apps must be replaced, and
2. To facilitate a transition, Flask-Lastuser must continue setting g.user in before_request until the transition is complete.

[jace]

The current_user implementation has moved to hasgeek/coaster#154. This ticket can be closed as all additional detail is now covered in #29.

[jace]

Coaster's corresponding tickets (hasgeek/coaster#154 and hasgeek/coaster#155) have implemented this as a current_auth proxy. Lastuser's switch is in hasgeek/lastuser#229.