xeditable: proof.note, filepicker+aviary: image

This PR is quite long and not completed.

The x-editable for editing a proof.note is done.
The filepicker + aviary to edit uploaded screenshot is not implemented yet.

Working on this allowed me to do a lot of refactoring and simplify views.
I have reached a point in code abstraction where I finally found out how to
simplify the user - proof - requirement relationship.

This commit diff shows the huge refactoring and the simplified code which is
the result of the hard work.

Summary of refactoring:

-  workflow is accessible at /checklist url
-  requirement resource has been reduced to the minimum: it's basically a
dictionary for the proof resource.
-  proof resource is the new hero (we should have done this from the start)
-  user.proofs for each requirement are instantiated at user creation (with a
check for inexistant proofs at user sign in for legacy users)
-  proof#index lists proofs grouped by requirement parent (1, 2, ... A)
-  ProofsController gets all the functions of the RequirementsController: index,
update 
-  PDF report generation has been fixed to work with user.proofs
-  all those messy methods 'requirement.proof_for(user)' and similar are gone,
 proof instance gets the importance which deserved from the start
-  specs are failing for the moment (I'll fix them)
-  some more refactoring is needed (the progress_bar generation is messy)