Downgrade externalized modules to a version that doesn't hold BC-breaking Webmozart-assertions

* Externalize ldap

* Remove tests for LDAP-module

* Redirect old LDAP-class to ldap-module for backwards compatibility

* Externalize negotiate

* Install pecl-krb5

We need to include this in our travis-tests as long as we include the negotiate-module in the release

* Externalize consent module

* Update Psalm

* Ignore DocblockTypeContradiction and RedundantConditionGivenDocblockType

* Fix Psalm-errors

* Raise minimum PHP-version to 5.6

* Update lock-file accordingly

* Fix phpunit

* Suppress Psalm-issue

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.