Replace mailhog with mailpit

[mattporritt]

mailpit (https://github.com/axllent/mailpit) is a more maintained mail test client with a straightforward api.

Fixes #257

[mattporritt]

This replaces mailhog with mailpit. It is more or less a drop in replacement.

[skodak]

this is a very annoying change, please do not change the contents of config.php unnecessarily, and if you do then please use more future-proof setting and docker names such as:

$CFG->smtphosts = 'mail:1025';