Slack: Add support for Workflow Keyword notifications in Slack integration

[SirLouen]

Closes: https://meta.trac.wordpress.org/ticket/8157

This is an improvement for the current version of ticket_component_filters but targetting Workflow Keyword changes in the firehose.

When it detects in Changes for the "new" keywords and the diff between old and new is different, it will trigger the send from \Dotorg\Slack\Send

The advantage of this trigger, is that it's constantly parsing each new comment in the firehose, not only new tickets as in ticket_component_filters. Ideally also the rest of the triggers should be migrated to this system, because focuses and keywords can be introduced anywhere within a ticket, not only in the ticket OP.

This is specially true for testing (needs-testing) workflow keyword, that its rarely added in the first ticket, as is a intermediate keyword that its added when people actually need testing.

I have not added any placeholder texts, and I'm just using the default message in \Dotorg\Slack\Send. But maybe in the future we could set some hardcoded messages in the code, maybe as an associative array, to customize a bit the thing.

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Props sirlouen.

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