Allow user to be assigned to escalation rule

...move_null_constraint_on_schedule_from_escalation_rules.rb

db/migrate/20210722074242_add_user_to_escalation_rules.rb

...rate/20210722074256_add_user_index_to_escalation_rules.rb

db/migrate/20210722074309_add_user_fk_to_escalation_rules.rb

...22074339_add_xor_check_constraint_for_escalation_rules.rb

db/schema_migrations/20210722074220

db/schema_migrations/20210722074242

db/schema_migrations/20210722074256

db/schema_migrations/20210722074309

db/schema_migrations/20210722074339

db/structure.sql

doc/api/graphql/reference/index.md

...l/mutations/incident_management/escalation_policy/base.rb

...l/types/incident_management/escalation_rule_input_type.rb

...graphql/types/incident_management/escalation_rule_type.rb

ee/app/models/incident_management/escalation_policy.rb

ee/app/models/incident_management/escalation_rule.rb

...s/incident_management/escalation_policies/base_service.rb

...incident_management/escalation_policies/create_service.rb

...incident_management/escalation_policies/update_service.rb