Add ability to add multiple pieces of evidence to a given standard annotation

[vanaukenk]

@tmushayahama can we add this ability at the point of annotation creation as well as in the table to an existing annotation?
At the point of annotation creation, we could do this in a similar way to add another annotation extension or comment.

From @krchristie :

Considering that Noctua has allowed, even encouraged, adding a second piece of evidence to the same annotation for years, I think the best solution would be to add this functionality to the Standard Annotations editor as well. This would certainly produce the best work flow for annotators as it is quicker to add an additional piece of evidence to the same data that is already in the form than to have to reenter all of the information in order to add additional evidence.
#49

[thomaspd]

Standard annotations in GO have always had one piece of evidence. I think we need to follow that specification here, so we should not make this change. If the evidence is different, it should be a separate annotation.

[vanaukenk]

@thomaspd - Just to clarify:
This ticket isn't about changing the standard annotation specification, but rather allowing curators to add multiple pieces of evidence at the time of creating an annotation just to be more efficient. Each piece of evidence would still be modeled as a separate annotation as we always have.