[API] Add endpoint to promote a given waitlisted applicants

[taesungh]

We'll be accepting a select list of waitlisted applicants both during an additional release wave and for walk-in hackers during the event. We'll need an endpoint accessible by both reviewers and the check-in team to facilitate this.

• Add an admin endpoint to promote a waitlisted applicant
  • The applicant can be specified as a path parameter
  • Convert applicant status from WAITLISTED to ACCEPTED
  • Send the applicant the waitlist release email from #292