New activity/session being added, activity date restricted by event dates

[brettski]

Is your feature request related to a problem? Please describe.
When a new activity / session is added the date of that activity must fall within the start and end dates of the event it is being added to. For the website this means that only dates and times between the start and end dates should be selected. For the Online only events I am thinking it is okay if the total time for the session extends past the event's end date as long as it starts within the start and end times.

Describe the solution you'd like
When creating a new activity the selectable dates to schedule it should fall within the start and end date of its event.

Describe alternatives you've considered

Additional context

related to api issue: ThatConference/that-api-sessions#123