Add help to creation form how to define license

[nuest]

When I upload an Rmd file with incorrect license definitions, I should get a helpful hint how to define the license correctly:

Incorrect:

license:
  code: Apache License 2.0
  data: Creative Commons Attribution Share-Alike 4.0
  text: CC BY 4.0

Correct:

licenses:
  code: Apache-2.0
  data: CC-BY-SA-4.0
  text: CC-BY-4.0

Maybe a link to https://spdx.org/licenses/ in the warning?

Can users use both the identifier or the full name?