154 add challenge properties for header

[tschaffter]

Fixes #154

Notes

• Limited validation is performed (e.g. the platform ID submitted when creating a challenge is now validated to check if it belongs to an existing platform). Extra validation will be added in a separate PR.

[tschaffter]

Here are an example of challenge returned:

{
  "description": "This challenge is awesome.",
  "displayName": "Awesome Challenge",
  "endDate": "2020-12-31T00:00:00Z",
  "fullName": "awesome-user/awesome-challenge",
  "id": "6138f253137b9b4fbf247290",
  "name": "awesome-challenge",
  "ownerId": "6138f250137b9b4fbf24728f",
  "startDate": "2020-11-10T00:00:00Z",
  "status": "active"
}

[coveralls]

Pull Request Test Coverage Report for Build 1214301528

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 29 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 46.463%

---

Files with Coverage Reduction	New Missed Lines	%
openapi_server/dbmodels/challenge.py	3	85.71%
openapi_server/controllers/challenge_controller.py	26	18.07%

Totals
Change from base Build 1199481596:	0.2%
Covered Lines:	1143
Relevant Lines:	2460

---

💛 - Coveralls

[rrchai]

Nice. Thank you