Make API URL depend on base_url setting

[marcus67]

Current status: 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 status: 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 https://github.com/marcus67/little_brother_taskbar/issues/42.