Updating portal dataset schema for external viewers

[anngvu]

The portal dataset schema defines how the card looks on the portal and what will be curated for a dataset. Currently, we want to provide links to external viewers where some or all of the dataset data are represented (cBioPortal, GEO, CELLxGENE, etc.). We have an option to

1. define a specific attribute per viewer, e.g. viewcBioPortal + viewGEO

• Perhaps clearer to users.

2. alternatively, something like externalViewer which will contain a list of accessions

• The simpler and more maintainable solution. There will be one-off viewers that are only relevant to one dataset, e.g. the data for Drug-Target Explorer has a bespoke Shiny app as the external viewer... Adding 20 different columns per viewer is not great.
• Perhaps too generic and therefore confusing to users.

More pros and cons...
@allaway @jaybee84 @cconrad8