Add issue_customer_relations_contacts linking table

What does this MR do and why?

Now we have Organizations and Contacts and are making good progress with the GraphQL CRUD endpoints... we need a way to start using them.

This MR introduces a linking table and association between contacts and issues.

Screenshots or screen recordings

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20210922215740 RAILS_ENV=development
== 20210922215740 CreateIssueCustomerRelationsContacts: migrating =============
-- create_table(:issue_customer_relations_contacts, {})
   -> 0.0095s
== 20210922215740 CreateIssueCustomerRelationsContacts: migrated (0.0096s) ====

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20210922220104 RAILS_ENV=development
== 20210922220104 AddIssueCustomerRelationsContactsForeignKeys: migrating =====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:issue_customer_relations_contacts)
   -> 0.0030s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issue_customer_relations_contacts\nADD CONSTRAINT fk_0c0037f723\nFOREIGN KEY (issue_id)\nREFERENCES issues (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0018s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE issue_customer_relations_contacts VALIDATE CONSTRAINT fk_0c0037f723;")
   -> 0.0024s
-- execute("RESET statement_timeout")
   -> 0.0006s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:issue_customer_relations_contacts)
   -> 0.0022s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issue_customer_relations_contacts\nADD CONSTRAINT fk_7b92f835bb\nFOREIGN KEY (contact_id)\nREFERENCES customer_relations_contacts (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0011s
-- execute("ALTER TABLE issue_customer_relations_contacts VALIDATE CONSTRAINT fk_7b92f835bb;")
   -> 0.0013s
== 20210922220104 AddIssueCustomerRelationsContactsForeignKeys: migrated (0.0278s)

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20210922220104 RAILS_ENV=development
== 20210922220104 AddIssueCustomerRelationsContactsForeignKeys: reverting =====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:issue_customer_relations_contacts)
   -> 0.0030s
-- remove_foreign_key(:issue_customer_relations_contacts, {:column=>:issue_id})
   -> 0.0037s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:issue_customer_relations_contacts)
   -> 0.0025s
-- remove_foreign_key(:issue_customer_relations_contacts, {:column=>:contact_id})
   -> 0.0032s
== 20210922220104 AddIssueCustomerRelationsContactsForeignKeys: reverted (0.0255s)

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20210922215740 RAILS_ENV=development
== 20210922215740 CreateIssueCustomerRelationsContacts: reverting =============
-- drop_table(:issue_customer_relations_contacts, {})
   -> 0.0030s
== 20210922215740 CreateIssueCustomerRelationsContacts: reverted (0.0060s) ====

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Related to #2256