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

I think, now that I have a better idea what this is about, I'm more convinced it will be nice to implement these transforms on top of the modeling framework, just, well, because :D.  I wouldn't bother for the purposes of this PR, and the composite modeling still needs to be fully merged, but then later a lot of deduplication of code could probably happen for this purpose.

In that case would it even be necessary for models to support the `clip` option (not that I'm against it)?  It just seems to me that the clipping can be done in `scale_image`.  Or is the idea to allow some specific transform implement clipping in some special way?