Create script that cleans up stale nginx symlinks

[rclarkburns]

From Slack chat:

We had to git-ignore the config/dev/nginx config file for a project so it got deleted
when I rebooted my computer, nginx didn’t start because of the missing config file.
That’s going to be a problem for people. They’ll delete projects in ~/src and nginx won’t start because symlinks point to missing files. We might want to write a fix nginx command or something that checks that the symlinks point to files, and delete the ones that don’t