Replaces method-injected application container with instance on BaseController.
Update to in-progress speaker-edits-profile feature.
Adds two functions to twig environment for resolving asset/upload paths.
Implements first pass at application service for retrieving speaker profile.

- Adds application service: Speakers
- Adds SpeakerRepository interface with Spot implementation (no integration test yet).
- Add exception that is thrown when an Entity is not found.
- Adds DTO that represents a read-only "speaker profile".
- Accompanying updates to Twig template for dashboard.
- Adds interface for authentication service.
- Integrates Speakers application service into DashboardController without controller service injection.
- Minor update to test method naming for Application container.