Untangle reading configuration from creating app object

[mvdbeek]

It would be nice if we could read Galaxy's config in (somewhat) external scripts (like #13383) and in gravity, without having to bring in galaxy.model and galaxy.web and all the things that dangle off of it.

In a first step that could just mean moving things that build Galaxy manager instances and the mapping object out of lib/galaxy/config/__init__.py and then publishing galaxy.config as a package.

Down the road it would be great if we could also read the job and workflow config via this package.

[hexylena]

I wrote something in gxadmin to do this as well, it's quite slow as is. I'd love to see this!