[0.4] Custom express instance no longer works

In 0.4, settings keystone.app or doing keystone.set('app', app) where app is your own express instance, prevents basic functionality (i.e. keystone admin) from mounting on the custom instance. This is a regression from 0.3.

The regression seems to be introduced here https://github.com/keystonejs/keystone/commit/2c2fab2641bec11616bfaa09fe4c8e2168a5fe94#diff-11403597a22b9bfded445849df6f58dbR48.

initExpressApp() returns the express instance instantly if one has already been set by the user. This means that the logic that mounts the admin routes (and database connections, middleware setup etc etc) never gets executed.

I am currently working around this by manually calling the createApp function:

keystone.app = app
keystone.start({
    onHttpServerCreated: function () {
        require('keystone/server/createApp')(keystone)
    }
})

[JedWatson]

Fixed by #2839

[JedWatson]

I think the documentation is out of date… this has been rewritten quite comprehensively, and tested in the keystone-test repo: https://github.com/keystonejs/keystone-test-project

See keystone.js, advanced.js and express.js for my test cases… if I have missed a test scenario, probably the best thing to do is add it there, and then PR keystone (or @mxstbr or I can help) to make it work.