Better onboarding

[benweatherman]

A few things would be nicer about the onboarding process

• Add intro copy to the settings dialog to say what's going on
• I don't think there's a reason to keep things in a separate branch anymore. We could still offer it as a config option, but maybe we should default to the master branch
• We could add an additional parameter for folder so it's not always stored in the githeartissues folder
• Enumerate available branches when you put in your repo. Add a loading state for that.
• Make the instructions for creating an orphan branch persistent