Allow index page in robots.txt

See merge request !1

This is necessary for, eg., centering part of the text…

Includes preparation.pdf rule

A dictionnary's order is not deterministic (eg. when switching python
versions), thus the previous protocol would often fail to authenticate a
legitimate request.

Poor Debian users.

`user` was not returned by configure_user

Close #6.

Whenever a user logs in through CAS, their username is looked up in the
.rhosts file provided through settings. If it is found, the user is
granted staff status and BOcal group (cf fixture).

The rights are re-evaluated at each login and at each access to
/admin/login.

Close #2.

cas.eleves.ens.fr allows users such as '  FooBAr ' or 'foobar' in
indifferently

So far, CAS-created users don't get any privilege based on rhosts

Note: the CSS is utterly horrible. Please change this asap.
Close #5