[sources] Simplify source's init method
Only call it when enabled instead of giving a boolean flag indicating whether it
is or not (which were not correctly considered).
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo