Add Slides Link to Events

[nik-dange]

Adds a field to Events specifying a link to an event slide deck.

[github-actions[bot]]

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[nik-dange]

[nik-dange]

Discussed and decided that this is probably not a necessary change until we have a better idea of how this would be used on the frontend

[nik-dange]

this pr is garbage. never convince me to create this feature again