clean up settings (bug 999774)

* merge settings
* group and then alphabetise them so they aren't all over the place
* delete some of the ones that obviously aren't needed any more, this might break a few amo'y things, but whatever.
* left some of the less obvious or harder ones to remove, i confess.

omg, how are you meant to tell if this will still work?

Use this gist: https://gist.github.com/andymckay/2cb1d7673717496d0796

Then checkout trunk and run `manage.py dump_settings > trunk.json', repeat the same with this branch `manage.py dump_settings > 999774.json` then `diff -u trunk.json 999774.json` and you'll see what got deleted and what changed.