Allow users to mark events as conference events.

[lpil]

• New enum field in prismic
• Field exposed in badger brain
• Test sanitizeEvent

Test plan

• Edit an event in prismic, selecting an eventType for it from the drop down select.
• Query badger-brain for the event, including the eventType field. The value you picked should be present. Events without a value (i.e. they are set to "other") return null for this property.

[pixelblend]

LGTM 👍

[lpil]

Thanks!