Add extras field to EventAgendaItem

[patcon]

As discussed here, Toronto has timestamp data for individual agenda items:
opencivicdata/scrapers-ca#163 (comment)

My thoughts is that this would fix best into an extras field on agenda items. In the meantime, I could put it in Event extras, but that doesn't seem ideal.

Thoughts?

cc: @jpmckinney

[jpmckinney]

Is there no extras field on agenda items?

[patcon]

Nope, not seeing it in the db table

[jpmckinney]

I suppose that's the only place to put it then.

[jpmckinney]

Reticketed.