Discopower fixes

Also move the templates to the "saml" module, since they belong in there.

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.

metadata: Make the documentid a hash over the document contents, so it does not change on every page load

Fix Psalm issues lib/

Also reorder alphabetically

Add an extra build step to specify the current version (the one we are building) in the composer.json file.

Fix Psalm issues modules/

According to Sebastian Bergmann: "That configuration setting has not had an effect in years. It was removed long ago".

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

Make metadata validateFingerprint algorithm configurable

Some metadata publishers offer SHA256 fingerprints to validate the
fingerprint of the certificate used to sign metadata. Allow users to
configure the fingerprinting algorithm to facilitate validation.

For backward compatibility, the default algorithm remains SHA1.

Factor out SigningTestCase for reuse

Supress warning on absent NameIDFormat

In https://github.com/simplesamlphp/simplesamlphp/pull/972 we moved the
phpunit config to the root of the project. This was not updated in the
testing documentation.

HHVM is not compatible with Symfony4 and it is no longer worth the hassle

Allows sharing a valid certificate setup with SignerTest. In SignerTest,
reuse good certificate as the main certificate for signature and add a
second certificate for testSignWithMultiCertificate.