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.

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

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

This reverts commit b9f92f33.

This reverts commit 82a46a7c.