Split Notifications Listeners into Mail/webhook ones

[uberbrady]

This further breaks out our various Listeners for Notifications - so that there's one listener that does Webhook stuff, and one that does Email stuff.

So, effectively, now we have four individual listeners, where we used to have just one:

• CheckoutableCheckedInEmailNotification
• CheckoutableCheckedInWebhookNotification
• CheckoutableCheckedOutEmailNotification
• CheckoutableCheckedOutWebhookNotification

This should make it easier to separately troubleshoot each and any of these separate notification events. There is a little bit of 'magic' around the $checkoutAcceptance property of the event itself, which is done to prevent us from inadvertently creating two checkout acceptances in the case where both the Webhook and the Email are both firing. But, in general, I feel like it's a lot clearer to reason about this way.

[codacy-production]

Not up to standards ⛔

🔴 Issues 6 medium · 2 minor

Alerts:
⚠ 8 issues (≤ 0 issues of at least minor severity)

Results:
8 new issues

Category	Results
CodeStyle	2 minor
Complexity	6 medium

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🟢 Metrics 69 complexity

Metric	Results
Complexity	69

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