Fix AttributeError for "lang" on repo/client connections This is a regression introduced by b48020a80dc3, which removed call to set_language from the `req._set_user` method. This is fine, but we still want a language on connections, so we've to handle it if necesary in Connection.__enter__ (i.e. once connection is properly open). When using pyramid front-end, which has a users cache, we've to cache its language as well because we must not access its preferred_language method since it's not bound to a proper connection.

Tutorials
~~~~~~~~~

We present a few tutorials of different levels. The blog building
tutorial introduces one smoothly to the basic concepts.

Then there is a photo gallery construction tutorial which highlights
more advanced concepts such as unit tests, security settings,
migration scripts.

.. toctree::
   :maxdepth: 1

   base/index
   advanced/index
   tools/windmill.rst
   textreports/index
   dataimport/index