Move Gatsby Admin build step to postbuild of Gatsby itself

[mxstbr]

Currently we have some issues with how Gatsby Admin is deployed. It's being build in pre-publish and this can cause weird behaviour eg when testing changes. Thus Gatsby Admin should be built/deployed differently.
I think however, we should keep the setup where we publish compiled artifacts. I do think that there is no reason to have separate package for that, instead admin artifacts should be included in Gatsby package itself.

Move to post-build of Gatsby
[...] that means we make this command be a post-build command for gatsby. That command should use currently compiled version of Gatsby.

• @freiksenet in https://github.com/gatsby-inc/core-team/issues/4