Add support for contacts autocompletion

app/assets/javascripts/gfm_auto_complete.js

...sets/javascripts/vue_shared/components/markdown/field.vue

app/controllers/projects/issues_controller.rb

app/helpers/application_helper.rb

app/models/customer_relations/contact.rb

app/services/issues/set_crm_contacts_service.rb

config/feature_flags/development/contacts_autocomplete.yml

doc/user/crm/index.md

doc/user/markdown.md

doc/user/project/quick_actions.md

ee/spec/helpers/application_helper_spec.rb

lib/gitlab/quick_actions/issue_actions.rb

spec/features/issues/gfm_autocomplete_spec.rb

spec/frontend/vue_shared/components/markdown/field_spec.js

spec/helpers/application_helper_spec.rb

spec/models/customer_relations/contact_spec.rb

spec/services/issues/set_crm_contacts_service_spec.rb