Make API URL depend on base_url setting

[marcus67]

Current behavior: The URL for the API calls (e.g. /api/status) is always relative to the root URL /. The configuration item [StatusServer]base_url is not used to compute the API URL.

Desired behavior: The API URL will be relative to the URL set in [StatusServer]base_url, e.g. with the base_url=/some-base the API URL for status requests will be /some-base/api/status.

Also see marcus67/little_brother_taskbar#42.