Support for Git submodules

[benhatsor]

These are imports to other repos within the current repo under a certain directory.
To detect these, check if the git_url is pointing to a different repository than the current, and if so, show a different icon and redirect on click. In Live View, they return a special file type submodule.
They're currently not supported in Filebrowser or Live View.

Submodule data is written to ./.gitmodules.