Drop gradient image, which can not be animated with css transition

This make links and button a bit different, and so, it is easier to know which element can open a new tab

Add some color just for prototyping

Reduce field padding.
Move home link.

It prevent accessibility, as focused elements are not visible

/reviewed-on nexedi/erp5!139

@Tyagov review, please. I'm creating a test suite now and will post about the test results as soon as they are available. 

- An environment object was implemented to help us deal with the multiprocess
architecture of ERP5 and objects that cannot be easily stored in the ZODB.
It stores definition of functions, classes and variables as string. The implementation
uses a dumb Environment class to allow users to make `define` and `undefine` calls,
which are captured and processed by an AST transformer before code execution.

- Along with the environment object, an automatic "import fixer" was created. It does
not allow users to import modules as they normally would, because this may cause
collateral effects on other users' code. A good example is the plot settings in the
matplotlib module. It will fix normal imports, make them use the environment object
mentione earlier automatically and warn the user about it.

Some bugs were fixed with this new implementation: 

- https://nexedi.erp5.net/bug_module/20160318-7098DD, which reports an inconsistency
on portal catalog queries between Jupyter and Python (Script) objects. Probably an
issue with user context storage in ActiveProcess

- https://nexedi.erp5.net/bug_module/20160330-13AA193, which reports an error related
to acquisition when trying to plot images, which happened in other situations, although
this is not officially reported in Nexedi's ERP5. This probably also was happening because
of old user context storage.


/reviewed-on nexedi/erp5!131

Variables are investigated, recursively in case of container objects (like lists, for example),
to detect if they can be stored in the ZODB.
In this investigation persistent objects are identified by being an instance of the object
class and implementing a `__getstate__` method that raises no exception. If the variable is
not a Persistent object then we try to pickle and load it.

While developing the pickleable object identification a complication was found. It seems that
the code cannot capture cPickle.PicklingError in the usual way, `except cPickle.PicklingError`.
It's consequence of some weirdness with regards to pickle/cPickle modules exceptions classes and
more about it can be read at http://bugs.python.org/issue1457119. So, the workaround for this complication
was to catch all exceptions and check the exception class name as string.

The whole check for zodb persistence was moved into an utility function for the sake of readability
and code maintenance.

The Base_executeJupyter script object was transformed into an extension to be able to properly handle
transaction errors and render them correctly inside Jupyter.

Avoid getting all related values when we jump to a module.

Getter of content translations were setting an empty translation dict. So
there was ZODB pollution by doing useless changes.

erp5_web_renderjs_ui_test: Isolate testRenderjsLogout from other tests, otherwise next test fails because of unauthorized error.

erp5_ui_test_core: Don't use Base_doLanguage. Base_doLanguage opens another selenium test runner in the bottom frame of the original selenium test runner by redirect and another selenium test runner does the same thing recursively.