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.
In light of Emacs packaging improvements such as those mentioned
in #11503, and with the addition of a systemd service (#15807
and #16356), and considering that the wiki page is completely
out of date (#13217), it seems that some documentation is in order.
