Re-enable localisation of flash messages after migration to Flask 0.10+

Flask 0.10+ makes it impossible to use `login_manager.login_message = lazy_gettext('Please log in')` (no more pickle), thus buggering localisation of `flask-login`'s flash messages.

Add a `localize_callback` attribute to `LoginManager` that, if present, will be called with `login_message` and `needs_refresh_message` and whose return value will be sent `flash`.