Validate all mappings

[visola]

User Story

As a user I want to know if my mappings is wrong as soon as possible.

Acceptance Criteria

• [ ] Load mappings during startup
• [ ] Add a framework to create validators for mappings
• [ ] Execute the validation when loading the mapping
• [ ] Make sure it works with #7
• [ ] Expose validation errors in the Admin UI

[visola]

This will need to be reworked after merging branch one to master.