Add feature to disable pull requests on a repository (2016)

Like disabling Wiki and Issues in the project's Settings view I would like to be able to disable pull requests on a repository.

This will be very useful for projects which have only a mirror on GitHub and they DON'T want to use GitHub's pull request mechanism for accepting contributions.
Or a project open sourced by a company is in the process of accepting GitHub pull requests but not yet, they still have some work to do internally before they are ready. (e.g. Google's TensorFlow project used to not accept pull requests through GitHub at the very beginning, but now they do)

Mirror examples:

• Linux kernel - 92 people wasted their time sending pull requests because they haven't read how to contribute to the Linux kernel guide
• Qt project - GitHub is only used as a mirror and not accepting contributions through it, they use their own Gerrit instance
• kdeconnect-kde - GitHub is only used as a mirror and not accepting contributions through it
• kdeconnect-android - GitHub is only used as a mirror and not accepting contributions through it

And many others... the maintainer(s) of the project have to tell people every time that submitting pull requests through GitHub is NOT the way to contribute to the project, and to keep telling them to read the contributing guidelines and close their pull request:

• Fix for platform socklen_t on other C libraries than glibc. qt/qtbase#6

Having the option to disable pull requests on a project would solve this problem.