Make url fields mandatory

[AdrianDAlessandro]

• ROR does not exist for several providers, so must be optional.
• URL for providers: some of the providers are humans, not institutions/publishers (only one instance so far, where the learning resource is an academic paper). We could make publisher URL mandatory and suggest an ORCID URL for the human publisher. See what @anenadic thinks.
• URL for learning resources: some of these are generic, such as 'often offered by universities', where there is no canonical URL for a resource because it will be different for each learner's employer. So I would argue non-mandatory here... otherwise, the URL is not useful for those resources... or we take such resources out. Again, see what Aleks thinks too.

Originally posted by @PhilReedData in #580 (comment)

I'd lean towards making URLs mandatory for both. Thinking from a UX point of view, if somebody is looking a list of learning resources, they would expect to be able to click a link to take them to that resource. Same with the providers, there would be an expectation of some kind of reference.

Originally posted by @AdrianDAlessandro in #580 (comment)