Update and make the annotation schema more uniform.

[manthey]

Resolves #762.

[naglepuff]

I'm having some trouble with these changes. Here's an annotation that works (can be successfully uploaded) in master but not in adjust-schema:

{
	"name": "overlay_test",
	"description": "",
	"elements": [
		{
			"type": "imageoverlay",
			"girderId": "61b243b9b9de8887df5cd186",
			"opacity": 0.5,
			"group": "myGroup"	
		}
	]
}

In this case its the group property that appears to be the cause of failure. This seems to be the case for label, user, and id.