Prevent manual broadcast button from appearing when one exists

[xmedr]

Overview

This is a fix for a bug in pr #1112. On staging, the logic for preventing a user from publishing another broadcast when one already existed was not working because the manual_events variable that the event template was expecting didn't exist. This sets that up properly.

This also disallows the option to publish a manual broadcast both for past events that already have a regular broadcast available, and cancelled events.

Testing Instructions

**Note: since that last PR applied the migration for manual broadcasts, this can now be tested on the review app (using the staging database)

• Publish an event
• Go to another event's page. Confirm that:
  • An event without a proper broadcast does not allow publishing a second one, and provides a link to the initial event
  • An event with a proper broadcast or a cancelled event does not allow publishing a new one at all

[xmedr]

whenever this gets approved i'll test on staging proper one more time before deploying