WIP: merge imageutils into astropy core

This is a WIP, but is the PR to merge ``imageutils`` into the Astropy core package.

Notes:

* I have attempted to preserve the history but removed code that simply affected affiliated package infrastructure code.
* The branches and merge history *within* imageutils is lost since I rebased this, but I think this is actually less confusing this way. We just have a linear history for the package.
* I also updated issue numbers in the commit messages to always include ``astropy/imageutils``
* This does not include lacosmicx which can be added separately once the licensing is sorted out. Doing that separately will make reviewing this PR easier anyway
* It is marked as critical because this was assessed as 1.0-critical (not a critical bug though)

This is not ready for review or merging, and still needs updates to the docs, tests to pass, etc.

cc @larrybradley @cdeil @adonath @eteq @hamogu @ejeschke

There's nothing much more in the way of an "additional layer".    Most of these already map exactly onto the existing API for Models, so it's mostly just a matter of not creating separate APIs for essentially the same sort of purpose.  At the same time, I agree this is special-purpose enough that if doing so introduces any unnecessary complexity that it's not worth it.  But I think you overestimate the complexity of the Model framework.  Most of that apparent "extra layer" is bypassed in most simple cases.