Race condition in merge train removal toast notification

Problem to solve

The following discussion from !155692 (merged) should be addressed:

• @hfyngvason started a discussion:

  @mrincon @pburdette We just got a report on slack that this problem is still occurring.

  I suspect the problem is that a notification may fire

  • before the MR transitions to merged
  • but while the car is still in merging state

  In this case, the MR will have a merge train car, but not an index. We delete cars on removal, so I think it would be better to watch the car's presence (regardless of its state): If the car was present before, and is no longer present, then the MR has been removed from the train.

Proposal

1. Add a connection from merge requests to the new merge train car type backend. But note that the type is still under active development, so please mark the field as alpha ‼ (it's stable enough for internal use, but we don't want to commit to its structure just yet)
2. Decide whether to display the toast based on the presence of the car frontend