[MICROBA-1743]
* Use `LearningHeader` component which can display the course information (title, org, number) in the header to the learner (matching existing design).
* Refactor components to introduce a `PageContainer` component in order to support displaying course information in header.
* Refactor the BulkEmailTool component to take advantage of the new course metadata context provider.
* Add tests for new `PageContainer` component.
* Fix existing tests due to refactor.
* Add "Email" label above recipient selection checkboxes to match existing design.
