fix: Notification page breakages (#5236)

- Remove the cascading foreign key indexes
- Add migration to clean up existing objects

fixes: #4285

app/models/team_member.rb

@@ -14,11 +14,6 @@
 #  index_team_members_on_team_id_and_user_id  (team_id,user_id) UNIQUE
 #  index_team_members_on_user_id              (user_id)
 #
-# Foreign Keys
-#
-#  fk_rails_...  (team_id => teams.id) ON DELETE => cascade
-#  fk_rails_...  (user_id => users.id) ON DELETE => cascade
-#
 class TeamMember < ApplicationRecord
   belongs_to :user
   belongs_to :team