Add support for RFC6265bis SameSite cookie attribute

* Update simplesamlphp-theming.md

* Add theme.controller config setting

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

getString doesn't like booleans

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

Trailing slash is no longer required in baseurlpath as per 1.15.

This can be used by templates to load resources in different ways, either optimized for the developer or for a production environment.

This allows us to use this new filter to translate strings from a given array of translations, where every translation is indexed by its ISO 639 code. A new configuration option ('language' -> 'priorities') is available too to control the alternative languages that can be used instead of a given language, when the latter is not found. The filter returns null when no suitable translation is found, so that it can be combined with "default()" to set a default translation for a given string.