Support licenses without spdx ids; add scancode LicenseDB as an additional datasource

Hi there,

First of all, dependency-track is a great piece of software - thanks for your work.

### Current Behavior:
Actually only licenses with spdx ids are supported.

### Proposed Behavior:
Full support licenses without spdx ids (categorization, alerting etc.) - by adding them manually (with api/ui) and/or by using https://scancode-licensedb.aboutcode.org.

OSS Review Toolkit, for example, uses scancode license ids as an additional identifier: https://github.com/oss-review-toolkit/ort/tree/master/spdx-utils/src/main/resources/licenserefs

Best Regards,
Marco