Add warning banner

[nlhkabu]

Closes #3426

[nlhkabu]

To affect this change on the beta banner, we will need to update pypi-theme. PR here: https://github.com/python/pypi-theme/pull/24

[nlhkabu]

@ewdurbin or @di - any chance of a review on this? It's blocking a PR from a new contributor.

[ewdurbin]

Not clear to me why the tests are failing. Seems the frontend build is failing on travis:

Error in plugin 'sass'
Message:
    warehouse/static/sass/blocks/_notification-bar.scss
Error: Undefined variable: "$brand-color-dark".
        on line 40 of warehouse/static/sass/blocks/_notification-bar.scss
        from line 90 of warehouse/static/sass/warehouse.scss
>>   background-color: $brand-color-dark;