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improve handling ofindentation and trailing spaces

* Disable electric-indent-mode globally since it causes issues with
  electric-indent-mode in ruby-mode. We can re-enabled it on a case by
  case basis.
* Since we can no longer use electric-indent-functions as the hook for
  delete-trailing-whitespace, use instead post-command-hook.
* Since that breaks the spacebar, wrap delete-trailing-whitespace in a
  defun, delete-trailing-whitespace-after, that leaves alone
  everything before the point (so space, tab, etc work as normal).