whiltelist emails leaking fixed

@ mention of reviewers`

@Didayolo`

A brief description of the purpose of the changes contained in this PR.

• Whitelist emails were leaked from the API (http://localhost/api/competitions/2/). Now that is handled. Only admins can see whitelist emails
• Secret key is also removed for non-admins

Issues this PR resolves

• #1366 (closed)

A checklist for hand testing

• before deploying this change access http://localhost/api/competitions/1/ and check that whitelist_emails is there
• after deploying, you should not see whitelist_emails

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge