Reorder plugin installation

This way, you get the small plugins that provide the core app
administative commands early, and the plugins that pull big Docker images
last, so you can create and configure apps while you wait for Nginx to
download.
Add startup-deploy-all
Install git-apps right after buildstep

This makes it so pushes will build the app just as soon as it's
safe to do so. If we installed git-apps *before* buildstep, a push
could trigger the buildstep buildhook before the image is finished
downloading.

This does mean any apps pushed *before* buildstep is installed
will not be built: see plushu/plushu-buildstep#4.