fix: mark signals as already emitted by event bus

[Ian2012]

Description: This PR mark the signals emitted from the event bus as emitted.

See openedx/openedx-events#312 for more information

JIRA: Link to JIRA ticket

Dependencies: openedx/openedx-events#312

Installation instructions: List any non-trivial installation
instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

[openedx-webhooks]

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[openedx-webhooks]

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