fix: logical render error in agenda component

[SahilDahekar]

Description

• Logical render issue was due to talk.session being used as a key to render div. This resulted in the react not being able to identify which div in agenda to rerender and ultimately rendering the break divs again.
• Changing the key to talk.time which is unique to each break as well solved the issue.

Related issue(s)

Fixes #456

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