restore slackengine to 3008

[ITJamie]

it seems strange that some parts of the slack integrations were kept in core in 3008:

salt/salt/utils/slack.py

Line 4 in f4db526

.. versionadded:: 2016.3.0

salt/salt/modules/slack_notify.py

Line 4 in f4db526

.. versionadded:: 2015.5.0

but the slackengine's were not?

the original slack engine was in since 2016.3.0 and the (optional) replacement using the modern backend (slack_bolt) was added in 3006

[barneysowood]

There's definitely some inconsistency here - not sure why we'd keep the execution module but not also keep the state module.

I'd probably argue that it should all be removed and go into a salt extension - given that the slack api seems to change a fair bit and it hasn't really seen any maintenance recently.

@ITJamie - I see you have various issues about slack components, so I assume you use fairly heavily? Does the old (non-bolt) engine work anymore?

[ITJamie]

It does technically work if you have old api creds but the old slack apis are not getting updated. And new users cant set that up anymore i believe.

The bolt one works but has a few fixes im maintaining locally.

[dwoz]

If you want to create a PR adding it back I'm okay with it. Long term slack integration is the sort of thing I would prefer be maintained in a salt extension.