Fix SendGrid webhooks timezone treatment

[tienne-B]

Fixes #2600

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: tabbycat/notifications/views.py

Function	Unhandled Issue
post	AttributeError: type object 'datetime.datetime' has no attribute 'timezone' /{tournament_slug}/admin/notifi... Event Count: 376

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