AJAX endpoint for setting and retreiving config values.

- GET-attributes:
- key:str: Name of configuration value to retrieve
- Return-attributes(JSON):
- value: The value of the configuration key if it exists.

GET /webapp/AppConfig/?format=api&key=max_video_size_in_MB
HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

{
    "id": 7,
    "is_public": true,
    "key": "max_video_size_in_MB",
    "value": "50"
}