Fix next/prev speaker not working and display speaker count

[HugoGresse]

• fix next/prev speaker not working
• display current index near next/prev speaker count
• remove EventSpeakers for lazy loading stuff

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 010e11f):

• https://conferencecenterr--pr86-edit-next-speaker-nbjhw6ku.web.app
• https://apiopenplanner--pr86-edit-next-speaker-uzjcaz32.web.app

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