Astropy helpers

This PR updates the package template to use the prototype of astropy_helpers, distributed as part of the package template using the git submodule mechanism.

This PR accompanies astropy/astropy-APEs#5 which tracks the APE draft describing astropy_helpers, as well as astropy/astropy_helpers#1 which contains the initial draft implementation of the `astropy_helpers` package and the `ah_bootstrap` module, and astropy/astropy#1563 which is the analog to this PR, but for Astropy itself (that is, it converts Astropy to using astropy_helpers).

This PR should not be merged until the APE is finalized, and this PR has been updated to point the astropy_helpers submodule to the main astropy_helpers repository.