Custom license categories

[gabidobo]

This PR adds support for new user-defined categories, and moving licenses between the default categories.

Updates:

• there are now two types of licence categories:
  • build-in, "default" categories (Public Domain, Permissive, Weakly Protective, Strongly Protective, Network Protective, Uncategorized)
  • user-defined categories
• users can define categories in the license policy:

{
  "audit": {
    "licensePolicy": {
      "categories": [
        {
          "name": "Permissive",
          "licenses": ["CC-BY-3.0"]
        }
      ],
      "high": ["cat:Network Protective", "cat:Strongly Protective"],
      "moderate": ["cat:Weakly Protective"]
    }
  }
}

• default category names are reserved
• when using a default category name, users re-assign licenses from their default classification
• the example above moves the CC-BY-3.0 license from the default Uncategorized to Permissive
• within the default categories, a license can only be assigned to a single category
• a license can belong to multiple user categories
• user-defined categories can also be used to generate issues, via the cat:NAME reference under the proper severity array in the license policy
• licenses don't have types anymore, but categories and names - the default licenses.json db has been updated accordingly

Closes #98