Add support for RFC6265bis SameSite cookie attribute

* Update simplesamlphp-theming.md

* Add theme.controller config setting

Break with MS AD

This introduces a "theme.header" configuration option in order to achieve that.

Twig will always use gettext, legacy will use legacy. Twig does not
work with legacy so new ui would break if you would not also set this
option. Instead, just switch the system in use based on the usenewui
config variable.

Update config.php
Update header.php

This new feature allows the users to have a checkbox in the login form
which works in the same way than "Remember my username" but for the
organization selected in the dropdown

getString doesn't like booleans

SHA-1 is still supported but needs to be configured explicitly if you need it.

This is minor, but if a comma is required when this is enabled the template should reflect that.