Export schedule to XLSX!!

[HugoGresse]

• add schedule export to XLSX (and HTML?)

Using XLSX, and another library for the style, it export the schedule with correct time!

TODO:

• change file name
• export all days
• maybe try to merge identical cell?

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 9db3c7f):

• https://conferencecenterr--pr124-copy-schedule-xlsc-gq8d775s.web.app
• https://apiopenplanner--pr124-copy-schedule-xlsc-jgh28f79.web.app

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