Add UI for setting permalink format

[ForbesLindesay]

See https://ghost.org/forum/everything-else/7361-changing-url-structure/

[novaugust]

Hey @ForbesLindesay, I appreciate your enthusiasm for improving ghost :D We already have an issue open for this, #1631. It'd help us out if you visited the roadmap and searched the repository for existing issues before raising new feature requests. (Here's a tiny guide for making feature requests)
Cheers

[ForbesLindesay]

Thanks, I did try using GitHub's search, but that's frustratingly ineffective.