Add default license to project

[goodmami]

Currently each version of an indexed project has its own license. While it's possible for a project to relicense with a new version, I think it's very unlikely, so it would be more convenient to have a default license defined at the project level. E.g.,

config.add_project('ewn', 'Open English Wordnet', 'en', license='...')
config.add_version('ewn', '2019', url='...')
config.add_version('ewn', '2020', url='...')

Then wn.config.get_project_info() would use the version license if present and the default project license otherwise.