Add new reimport buttons to add CH event later on

[HugoGresse]

If you use OpenPlanner as your backend your the conference website, you may have created the event without using the ConferenceHall import button. So solve this issue, this PR add two new features:

1. Add option to attach the OpenPlanner event to a ConferenceHall event.
2. Add the option to, while you reimport speaker and sessions using the already present button, also import format & categories.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit b3fbb58):

• https://conferencecenterr--pr79-re-import-with-forma-eco6oonj.web.app
• https://apiopenplanner--pr79-re-import-with-forma-lz6macgz.web.app

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