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.

Also reorder alphabetically

Deprecation warning: require-dev.mikey179/vfsStream is invalid, it should not contain uppercase characters.

cURL is not strictly required, as checking for updates is done only if the configuration allows it. This commit fixes also a misspelling error (s/suggests/suggest/) and adds the rest of optional dependencies as suggestions.

Start migrating to SAML2 getters/setters

Required for integration with Symfony4 applications; closes #1003

This reverts commit 9e98109b.

This prevents a lot of -dev dependencies in composer.lock which are
not used otherwise and generate 'scary' warnings like "Package x is
abandoned".

Closes #718