emacs: 24.5 -> 25.1

This commit removes all references to emacs24 with the exception of
emacs24-macports. The two folders in `pkgs/applications/editors` named
`emacs-24` and `emacs-24` are consolidated to a new `emacs` folder.

Various parts in nixpkgs also referenced `emacs24Packages` (pinned to
`emacs24`) explicitly where `emacsPackages` (non-pinned) is more
appropriate. These references get fixed by this commit too.