Disable FlaskLogin flash messages
Disable FlaskLogin flash messages

Previously we've been ignoring flash messages from flask-login in our
flash messages template (see [`toolkit/flash_messages.html#L6`][1]).

We should instead be stopping flask-login from sending these flash
messages in the first place; setting `LoginManager.login_message` to
`None` should do this (see [`LoginManager.unauthorized()`][2]).

The ability to suppress flash messages in this way was introduced in
flask-login version 0.1.1. See pull request maxcountryman/flask-login#1.

[1]: https://github.com/alphagov/digitalmarketplace-frontend-toolkit/blob/master/toolkit/templates/flash_messages.html#L6
[2]: https://github.com/maxcountryman/flask-login/blob/0.4.1/flask_login/login_manager.py#L124-L177

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