Clarified work items FF naming

...s/javascripts/work_items/components/work_item_actions.vue

...ts/work_items/components/work_item_attributes_wrapper.vue

app/controllers/groups/work_items_controller.rb

app/controllers/groups_controller.rb

app/controllers/projects/incidents_controller.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/work_items_controller.rb

app/controllers/projects_controller.rb

app/models/group.rb

app/models/project.rb

config/feature_flags/beta/work_items_beta.yml

config/feature_flags/development/work_items_mvc.yml

doc/architecture/blueprints/work_items/index.md

doc/user/okrs.md

doc/user/tasks.md

.../frontend/work_items/components/work_item_actions_spec.js

...ork_items/components/work_item_attributes_wrapper_spec.js

spec/models/group_spec.rb

spec/models/project_spec.rb