Allow custom urls for the project license field

[raethlein]

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] New Feature
• [X] Feature Improvement
• [ ] Refactoring
• [ ] Documentation
• [ ] Other, please describe:

Description:

Allow custom urls for the license field in project definitions. The license name will be shown as "Custom-url License" in order to prevent long urls be rendered. Addresses #31

Checklist:

• [X] I have read the CONTRIBUTING document.
• [X] My changes don't require a change to the documentation, or if they do, I've added all required information.