New organisation of settings files

We reproduce what has been done here:
https://github.com/dissemin/dissemin

The following files can be found under `cof/settings/`
- `common.py`: the settings that are shared by all the environments we
  have + the secrets (see below).
- `dev.py`: the settings used by the vagrant VM for local development.
- `prod.py`: the production settings (for both www.cof.ens.fr and
  dev.cof.ens.fr)

There is also a notion of "secrets". Some settings like the `SECRET_KEY`
or the database's credentials are loaded from an untracked files called
`secret.py` in the same directory. This secrets are loaded by the common
settings file.

cof/settings/secret_example.py

@@ -0,0 +1,4 @@
+SECRET_KEY = 'q()(zn4m63i%5cp4)f+ww4-28_w+ly3q9=6imw2ciu&_(5_4ah'
+RECAPTCHA_PUBLIC_KEY = "DUMMY"
+RECAPTCHA_PRIVATE_KEY = "DUMMY"
+ADMINS = None