feature-9040 + feature-9042: return register state of attendee

[nnhathung]

Fixes #9040 #9042

Short description of what this resolves:

• register state of attendee

Changes proposed in this pull request:

• register state of attendee

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[ghost]

Please add the api-docs

[nnhathung]

Stats API for registration and stations [/v1/states?event_id={event_id}&attendee_id={attendee_id}]

• Request Method: GET
• Request:

    Header:
      Accept: application/vnd.api+json
    
      Authorization: JWT <Auth Key>

• Response 200 get success (application/vnd.api+json)

{
   "is_registered": false,
   "register_times": null
}

[nnhathung]

Hi @cweitat , @mariobehling please help me to review and merge it

[mariobehling]

Thanks. I wonder why the tests do not run. Website says "add Circle CI to config.yml.", but it was always there. Hm.

[nnhathung]

Hi @mariobehling , its all passed now.