Add SPDX metadata to SoftwareLicense

[wkoot]

Add SPDX License Identifiers and Expressions to the SoftwareLicense model, in order to facilitate SBOM input.
Investigate if using license-expression, similarly to spdx-tools and cyclonedx-python-lib, is feasible/useful.

[wkoot]

The type field originally included in the SoftwareLicense feature request #8 could've been used to document license type.
Since it was not specifically meant to hold license expressions, it'll be left as is - also saving complex migration logic.
The page using-SPDX-short-identifiers-in-source-files shows that we're dealing with quite a complex string composition.
For now I think that a multi-select or text field based on the short identifiers as listed on https://spdx.org/licenses/ should suffice.
An autocomplete/suggestion option is needed, choices dropdown is probably too restrictive due to the combinatory logic..