[Events] [BE] Multi-select dropdown for event list selection

[wesrowe]

STATUS NOTE: This is waiting on CMS team's implementation of a multi-select dropdown component.

Description

User story

AS A VAMC editor,
I WANT the ability to publish an event to multiple VAMCs in my System, and to the Outreach Events calendar
SO THAT we have a single source of truth for Events.

Engineering notes / background

Analytics considerations

Quality / testing notes

Acceptance criteria

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