global: simplification of licensing

This PR proposes to simplify Workflow licensing to use Revised BSD License version only.  I think it is not necessary to explicitly mention both Revised BSD and GNU GPL.  My motivation is:

- Revised BSD License is fully GNU GPL compatible; e.g. Workflow can be redistributed within GNU GPL projects;
- Revised BSD License is more permissive; e.g. anyone could take sources and modify and redistribute them under GNU GPL later, if they wanted;
- it is usually easier to get external contributions signed for a single-license project when compared to dual-licensed one;
- this would conform to usual Invenio software projects' licensing practices (i.e. using more permissive Revised BSD License for "smaller" libraries, and usual GNU GPLv2 for "bigger" projects);
- finally, using one license is simpler; "less is more".

@romanchyla Do you agree?