Enhance and fix DOAP project license support.

There were several problems with the previous mechanism for mapping
license types:

1.  The doap:license URI should point to an RDF item, but this is
    a broken link and is not human readable.

2.  The supported licenses was fixed to 'gpl3+' only.

3.  The liquid template is fragile for mapping list types and is
    not generative (e.g. gpl3+, lgpl2, ...).

Because of this, a project_license plugin was created that:

1.  Generates the correct RDF URI for the doap:license property.

2.  Links to the correct human readable URL for the license.

3.  Handles all supported DOAP licenses on
    https://github.com/edumbill/doap/tree/master/licenses.

4.  Supports generative names for gpl and lgpl licenses.