PSR-12 compliancy

This ensures that we don't break any existing modules by renaming a template from ".php" to ".tpl.php".

The old mail class uses the built-in `mail` function in PHP, and builds a payload by itself using hard-coded HTML and MIME-snippets. Let's use a library for that; PHPMailer has new dependencies and does all the heavy lifting.

re-add it and fix all templates that depended on it being unset

class_implements() returns class names without leading backslash

This resolves #1009.

We were echoing the result of require(), resulting in a '1' being printed during login

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

This function can be used to load an asset from a twig template, adding a "tag" parameter to it with a few dynamic bytes depending on the version of SimpleSAMLphp, or the last modification time of the file if we are running master. This behaves as a cache-buster, forcing browsers to reload assets when a new version of SimpleSAMLphp gets installed.

This way we can forget about updating the copyright notice every year.

Create a new "configpage" hook for those modules that need to inject anything in the configuration page.

This allows us to change the signature of the hook, so that we pass it the \SimpleSAML\XHTML\Template object and then the hook can not only add data to be passed to the twig template, but also add its own translation domain. This was needed because many modules were putting their translations inside "core", and when moving those to the modules themselves, the new translation system cannot perform the translation because only the translations for the current module in use are loaded (in the config page, that's the core module). Therefore, we needed a way to get the Localization instance from the template, and a way to pass that to the hook so that the hook can register the translations for its own module.

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.

This is because some templates, e.g. errror, expect to be able to
include them. This fixes e.g. the No State error page.

This is useful when testing.

*    Updated the phpDoc comments to match the code
*    Added return statements
*    Added return values

It was deprecated in Twig 1.x and removed in 2.0.

It was deprecated in Twig 1.x and removed in 2.0.