[#7309] don't show uninstallable tools on /*/add_project

Factor out _installable so it can be called without an App.  Constructing
an App wasn't happening before, so the check was invalid.  And
constructing an App requires a Project and AppConfig

Also change `fields` from a class to a property so it doesn't run on import
[#7309] ensure c.project is set for all nbhd methods
[#7309] fix installable check for bootstrap too
[#7309] better place to set c.project for nbhd controller