T54204 handled the pasted and typed URLs, and *pasted* ISBN/PMID/RFC links, but not *typed* ISBN/PMID/RFC links.
That's what this task is for.
For pasted links we punt over to Parsoid to fill out the proper URL, etc, but the Sequence stuff is sync, not async, so punting to Parsoid is probably not the right answer. There might be a little bit of DRY-violation necessary, but it shouldn't be too bad.