Prototype multiple work types: Basic and ETD

[kerchner]

ETDs may need to have unique behavior, distinct from other GWSS items. Review the User Stories to validate this and articulate what that different behavior may need to be. Since workflows are likely to be part of how ETDs behave differently, we'll need to also consider how workflows are projected to work in Sufia 7.3, and we'll also need to assess to what degree the Sufia UI's rendering of multiple work types is ready for prime time.

In the meanwhile, we can create a prototype to better learn how this works. Explore:

0. Do nothing, in case implementing above-mentioned behavior is entirely doable without multiple work types.
1. Derive a child ETD type from the base work type (Dependency on Consider refactoring Work model name #45, but can still be done in prototype-y mode even without Consider refactoring Work model name #45; it would likely entail duplicating the work later in another branch but with the new model name)
2. Create a second base work type that would be a "sibling" to the base work type.

Test the UI and other functionality in each case.

This work should be kept in a separate branch, because our plan is first to release GWSS 2.0, which will be an upgrade of the existing app, and migration of the current content, to PCDM-based Sufia 7. Then, later, we'll release GWSS 2.1 (perhaps 3.0) to include ETDs.

This work requires #49.

[kerchner]

Prototype options for ETD work type (Github issue #47)