Cleanup zshrc

* remove duplicates with zlogin
* order by functions first, then bindings, then exports
* lowercase `setopt` arguments for consistency with `setopt` output
Add Git credentials.
Initialise rbenv on startup.
Add Homebrew directory to PATH.
Move personal details to local files.
Add Github Enterprise settings.
Add Github CLI.
Look for GitHub Enterprise settings.
Initialise rvm, if it's installed