Adds `--hide-ui-dir-config` flag to disable editing directory configs from the web UI. This can be set to prevent users from setting the directory to somewhere they shouldn't, for public (or semi-public) interfaces.

Directories are still read from config.json, so the server admin can still set them in the web UI and then relaunch with the hide flag, or edit the config manually.

Also:
- fix OptionInfo `component_args` keyword argument not being read if `component` isn't also set
- ensure that hidden settings aren't still read from the web UI (otherwise they could still be changed by tampering with the interface)