Add StudyGroup to submissions and show it for submissions and RfCs

Also take care of deleting a StudyGroup for existing submissions

app/models/study_group.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
 class StudyGroup < ApplicationRecord
-  has_many :study_group_memberships
+  has_many :study_group_memberships, dependent: :destroy
   # Use `ExternalUser` as `source_type` for now.
   # Using `User` will lead ActiveRecord to access the inexistent table `users`.
   # Issue created: https://github.com/rails/rails/issues/34531
   has_many :users, through: :study_group_memberships, source_type: 'ExternalUser'
-  has_many :submissions
+  has_many :submissions, dependent: :nullify
   belongs_to :consumer
 
   def to_s