This CL makes it easier to install navi on multi-tenant Linux systems, by allowing to provide pre-install cheatsheets in shared folders (and potentially pre-installed config in shared folders).
This does not break or change the existing behaviour. The shared directory / config file is only used if
- There is not already a directory / config file in the user XDG location
- Navi is compiled with NAVI_CONFIG_FILE and/or NAVI_CHEATS_DIRECTORY env var set
- The paths defined in above env vars exist
