Use config file for redbot_cli

[JannisBush]

Currently, redbot_cli uses a fixed config that is not easy to change.
The changes make redbot_cli use the same config file as the other services.

[mnot]

I suspect it'd be better to use $XDG_CONFIG_HOME, defaulting to $HOME/.config as per the spec.

Something like:

Suggested change

	config_parser.read(os.environ.get("REDBOT_CONFIG", "config.txt"))
	home_dir = os.environ.get("HOME")
	config_dir = os.environ.get("XDG_CONFIG_HOME", f"{home_dir}/.config")
	config_loc = f"{config_dir}/redbot.cfg"
	if os.path.exists(config_loc):
	config_parser.read(config_loc)
	else:
	config_parser.read_dict({"redbot": {"lang": "en", "charset": "utf-8"}})

(not tested)

[JannisBush]

I copied how to read the config file from https://github.com/mnot/redbot/blob/main/bin/redbot_cgi.py#L82
I guess both ways are fine, but it is probably least confusing if all files use the same mechanism to read the config file.

[mnot]

redbot_cgi.py is no more, so that shouldn't be an issue any more.