Move instigating source out of contributing paths (#253)

* Refactor contributing_paths into contributing_sources, deleting project node sources

* Instead of changing contributing_paths, add instigating_source

* Remove InstanceMetadata::project_node

* Stop pushing project path to front of contributing_paths since it doesn't matter now

* Remove accidental UI change for path display

src/snapshot_middleware/snapshots/test__project_path_property_overrides.snap

@@ -6,8 +6,8 @@ snapshot_id: ~
 metadata:
   ignore_unknown_instances: true
   contributing_paths:
-    - /foo/default.project.json
     - /foo/other.project.json
+    - /foo/default.project.json
 name: path-property-override
 class_name: StringValue
 properties:

src/snapshot_middleware/snapshots/test__project_with_path_to_project.snap

@@ -6,8 +6,8 @@ snapshot_id: ~
 metadata:
   ignore_unknown_instances: true
   contributing_paths:
-    - /foo/default.project.json
     - /foo/other.project.json
+    - /foo/default.project.json
 name: path-project
 class_name: Model
 properties: {}

src/snapshot_middleware/snapshots/test__project_with_path_to_project_with_children.snap

@@ -6,8 +6,8 @@ snapshot_id: ~
 metadata:
   ignore_unknown_instances: true
   contributing_paths:
-    - /foo/default.project.json
     - /foo/other.project.json
+    - /foo/default.project.json
 name: path-child-project
 class_name: Folder
 properties: {}

src/snapshot_middleware/snapshots/test__project_with_path_to_txt.snap

@@ -6,8 +6,8 @@ snapshot_id: ~
 metadata:
   ignore_unknown_instances: false
   contributing_paths:
-    - /foo/default.project.json
     - /foo/other.txt
+    - /foo/default.project.json
 name: path-project
 class_name: StringValue
 properties: