Integrate with other Slack Notification plugin

[swissspidy]

For #48 we need some more powerful Slack notifications. At least attachments would be helpful. And buttons. And unicorns.

We currently support https://wordpress.org/plugins/slack/, however that plugin is a bit limited in terms of available formatting and hasn't been updated in quite some time.

https://wordpress.org/plugins/dorzki-notifications-to-slack/ looks like a promising alternative.

For the time being we could support both, with the new one being the preferred solution for some more powerful Slack notifications.

Integrations seems to be pretty straightforward, see https://github.com/wearerequired/required-jobboard/pull/12 and https://github.com/dorzki/Slack-Notifications

[swissspidy]

Both plugins haven't been updated in many years. Yikes.