Ability to make the window shorter than 600px

On macOS, I can't make the window height any shorter than 600 pixels. It would be useful to make it shorter than that since it currently takes up more space than I'd like to keep it organized. I have Keybase open a lot of the time and so having it fit into its own area and be visible while still letting me work on other stuff is pretty important to me.

The issue names macOS window sizing but no file or test. Start by locating the macOS client entry point that enforces the minimum window height; done means the window can be resized below 600 pixels while remaining usable.

Written by the indexing model from the issue text.