Remove static MailHandler service methods

Problem/Motivation

The MailgunHandler class provides a bunch of static helper methods, like 'status', 'checkLibrary', 'checkApiSettings' and 'validateKey'.
They work fine, however, static methods may not be easily mocked, replaces by service decorators etc.

Proposed resolution

Create new non-static versions of 'status', 'checkLibrary', 'checkApiSettings' and 'validateKey' methods; replace any usages of static \Drupal with DI.
Replace static methods implementation with:

\Drupal::service('mailgun.mail_handler')->METHOD_NAME($arguments);

Deprecate static methods.

Remaining tasks

Implementation.

User interface changes

None.

API changes

A bunch of public static methods will be deprecated but will still work, so no API breakage here.

Data model changes

None.