Add GET /config API

[kyasbal]

Background

KHI frontend code relied on build time environment variable to change some configurations on frontend. The viewer mode configuration is an example, which can be enabled on backend CLI parameter, but frontend needs to be built with an environment variable.

Related #42

Solution

Add GET /config API to return current configuration needed for the frontend. This API will be called at the beginning of frontend code being loaded.
This API doesn't receive any request parameter, and response is just a map and we may add additional fields on the map later.