Share as SPDX

SPDX is a great way to put data into software component management systems. Given that ClearlyDefined has lots of interesting data and allows users to manage lists of components, it makes sense that a user be able to Share this data in SPDX format. There are multiple aspects to this

1. Need a mapping to/from ClearlyDefined and SPDX fields and values
2. Service code that actually formulates the SPDX syntax (see https://github.com/clearlydefined/service/issues/14)
3. UI affordance that allows users to share the SPDX
* This should be on the Definitions page to get document (or set of documents) for the list of components in the list
* On the full details page to share the SPDX for just one definition

@tsteenbe and @kestewart for awareness