Use standard annotation properties and structure for definition source

[cmungall]

Originally posted by @cmungall in #1472 (comment)

For definition sources:

• should be as axiom annotation on the definition rather than term-level IAO:0000119
• should be to a URL rather than descriptive text

See https://github.com/EnvironmentOntology/envo/wiki/A-guide-to-annotations-on-ENVO-classes-and-relations for ENVO-specific docs

@turbomam was #1472 created with robot? If so we will need to revise template

[turbomam]

No, I created the classes and annotated them in Protege

[turbomam]