overhaul repo_icon so there is no error if examples/ dir cannot be found, e.g. with `fdroid init`

There are so many possible installation paths for Python modules, it has been very hard to even find and test them all. This adds a fallback option if the examples dir cannot be found. A repo can work without an icon or the example config.yml.

This removes the fake assumption that the icon can be a full path in the config.yml. While the path was being properly passed through to the index file, the file was never copied properly into place nor rsynced to the web server.