Enhance DejaCode product attribution support for custom templates

It would be very useful to enable a DejaCode superuser administrator to make a copy of the standard attribution template and style sheet used by DejaCode and modify it to meet specific business requirements. Such a feature might include:

- the ability to identify the default attribution template for a dataspace
- the ability to identify a specific attribution template for a Product
- the ability to select a specific attribution template, other than the default, at attribution generation time
- a standard place to store one or more attribution templates to make selection easier
- documentation (how-to, tutorial) to explain the concepts and capabilities of attribution templates in DejaCode and how to customize them, and how to identify the product and product inventory attributes that are available for attribution
- retaining the existing "legacy" attribution template to support existing customers

Ideally, this enhancement would use much of the same code as ScanCode.io (SCIO), such as:
- Use the same CSS style as SCIO
- Full "pkg:" purls displayed to identify packages
- Render the licenses, in expressions and lists, using the SPDX ID (ie: SPDX key or "LicenseRef-dejacode-{key}"
- When available, provide licensedb_url in the license details (note that specific customer dataspaces will probably include local license definitions that are not in the LicenseDB).

The documentation should also explain the differences between product attribution documents and SBOMs, including the purpose of each.