Add user to testrun

* We want to identify a user that has triggered a testrun. Previously (in regular operation), only submission author who were regular users were able to start a testrun. Now, we want to prepare a future where submission authors are programming groups. Still, a testrun is triggered by an individual user and not a group.
* Further, this commit fixes some missing foreign key constrains.

app/models/programming_group.rb

@@ -6,6 +6,7 @@ class ProgrammingGroup < ApplicationRecord
   has_many :programming_group_memberships, dependent: :destroy
   has_many :external_users, through: :programming_group_memberships, source_type: 'ExternalUser', source: :user
   has_many :internal_users, through: :programming_group_memberships, source_type: 'InternalUser', source: :user
+  has_many :testruns, through: :submissions
   belongs_to :exercise
 
   validate :group_size