Merge branch 'ph/359977/attentionRequestFeatureFlagActor' into 'master'

app/controllers/projects/merge_requests_controller.rb

...phql/mutations/merge_requests/remove_attention_request.rb

app/graphql/mutations/merge_requests/request_attention.rb

...ql/mutations/merge_requests/toggle_attention_requested.rb

app/graphql/types/mutation_type.rb

app/helpers/merge_requests_helper.rb

app/models/concerns/merge_request_reviewer_state.rb

app/models/merge_request.rb

app/models/merge_request_assignee.rb

app/models/merge_request_reviewer.rb

app/models/user.rb

app/serializers/merge_request_user_entity.rb

app/services/merge_requests/base_service.rb

...ervices/merge_requests/handle_assignees_change_service.rb

app/services/merge_requests/update_assignees_service.rb

app/views/layouts/header/_default.html.haml

app/views/projects/merge_requests/show.html.haml

app/views/shared/issuable/_assignees.html.haml

app/views/shared/issuable/_reviewers.html.haml

app/views/shared/issuable/_search_bar.html.haml