New admin interface to follow SaaS usage & fix error not always displayed when loading data

[HugoGresse]

• add a new UI to follow OpenPlanner usage, restricted to admins (me), changing the firestore.rules
• fix a bug making load error not stored, thus not displayed

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 6a3dbc0):

• https://conferencecenterr--pr120-admin-1-eoqbxzhc.web.app
• https://apiopenplanner--pr120-admin-1-5mpg56ki.web.app

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