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

Now I understand the reasoning, I see that it would be good for photometry. I can even think of other places where it would be good, so do think it has general use, but it would seem to make most sense as a package with different options. Hence, for now I think it would indeed be best to keep it in `photutils`. One further advantage is that I think this would be the only routine most logically put in `image` -- i.e., not including it here allows us to punt on what to call the module!