add legacy ids to submission type

the REST api doesn't expose these ids, so graphql originally followed
suit, but having these ids will make writing tests easier.

Test plan:
  * query for _id on submission objects

Change-Id: I2b09cf691672f7f98e5c5e92ea05d46df5b0af30
Reviewed-on: https://gerrit.instructure.com/160924
Reviewed-by: Jonathan Featherstone <jfeatherstone@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins

app/graphql/types/submission_type.rb

@@ -25,7 +25,7 @@ module Types
 
     alias :submission :object
 
-    # not doing a legacy canvas id since they aren't used in the rest api
+    field :_id, ID, "legacy canvas id", method: :id, null: false
     global_id_field :id
 
     field :assignment, AssignmentType, null: true

schema.graphql

@@ -719,6 +719,8 @@ type StudentSummaryAnalytics {
 }
 
 type Submission implements Node & Timestamped {
+  # legacy canvas id
+  _id: ID!
   assignment: Assignment
   createdAt: DateTime