Add foreign keys and an index to StructuredError(Attributes)

These changes will ensure a better data consistency and faster access. We further ensure that records should be deleted properly.

app/models/submission.rb

@@ -15,7 +15,7 @@ class Submission < ApplicationRecord
   belongs_to :study_group, optional: true
 
   has_many :testruns
-  has_many :structured_errors
+  has_many :structured_errors, dependent: :destroy
   has_many :comments, through: :files
 
   belongs_to :external_users, lambda {