As an event organizer, I want to be able to add other organizes to my event, so that they can also view and manage registration in the same way that I can.

Epic Lead: @cmelo

Release details:
Discussed at Feb 9 sprint planning:

• We will use a feature flag
• We do NOT need a deployment window for adding the feature flag
• We will need to schedule one or more deployment windows when we want to remove the feature flag and turn on the feature - it is a config change

Acceptance Criteria:

• Allow organizer to add other organizers to event registration by specifying their username
  • The text should read: "Add the usernames of other organizers to give them permission to manage this event."
• Allow organizer to remove organizer
  • There should be an area for the organizer to add in the usernames
• If a new organizer is added to an event, they have all basic organizer privileges, including being able to:
  • Edit event registration information
  • Cancel the registration of a participant
  • Open or close event registration
  • Delete event registration
• Should your username of the event creator be in the list?
  • Yes.
• Should you be able to remove yourself, as an event creator?
  • Every event should have at least 1 organizer. So you can only remove yourself if there is more than one organizer. If there is more than one organizer, you can remove yourself.
• If you're not the event creator, should you be able to remove the event creator?
  • No. The primary organizer name should be displayed separately and it should be non-editable by other organizers.
• Note that all organizers currently have the same rights, but we will looking into separate organizer roles in the future (T316138).

Note: we need to check how the widget works when there's a large number of organizers.

Proposed Design
Prototype
Design file

When there is at least one username	Warning message when the field is blank	Error message when the user tries to the submit form with a blank field