EP Merge: Address issue with EP400 contentions not being available immediately

[dfitchett]

Story

As a user of EP Merge I would like the EP Merge App to wait for the EP400 contentions to be added to the claim so that the merge request doesn't complete the job before they are added.

Acceptance Criteria

• Merge Jobs wait for EP400 contentions to show up
  • the request to get the contentions is retried at a rate of 2 seconds for up to 1 minute

Notes about work

During End 2 End testing it was discovered that the EP400 contentions are not available immediately upon receiving eh merge request from vets-api, but that there is some delay from the claim being established to the time the contentions are added to the claim. See results and discussion in this slack thread.