Add superevent labels to under the superevent_neighbours data model

Description of feature request

The superevent_neighbours field in the event data model contains the superevent information keyed on superevent_id. However, the information contains the following

[
  "far",
  "gw_events",
  "preferred_event",
  "preferred_event_data",
  "superevent_id",
  "t_0",
  "t_end",
  "t_start"
]

It would be helpful to have the superevent labels since they convey the state of the superevent.

Use cases

This helps the superevent manager know the state of the superevent without making a GET call. For example, in the MR for the superevent manager in gwcelery: emfollow/gwcelery!873 (diffs)

Benefits

This would help remove any GET calls to gracedb in the superevent manager.

Drawbacks

This change is backward compatible, so no breaking change.

Suggested solutions

Change in the data model for the superevent_neighbours