Move editor support to a different repo

[emberian]

We have an org now, might as well use it and clean up /etc.

[emberian]

Cc @brson

[japaric]

Also see #3756

[brson]

Previous discussion

[huonw]

Done, thanks to @steveklabnik:

• https://github.com/rust-lang/rust.vim
• https://github.com/rust-lang/nano-config
• https://github.com/rust-lang/gedit-config
• https://github.com/rust-lang/rust-mode
• https://github.com/rust-lang/kate-config