In order to support the `Enum` field type the `DbFieldType` enum has been converted
to an abstract class with an implementation class for each database field type.
Instead of shipping an `error/404.html` template, added a `@ControllerAdvice` that handles `DbAdminNotFoundException`.
This way, we only show our error page for exceptions that are thrown inside our controller's methods, and don't interfere
with the 404 settings for the other routes defined by the user.
fragment in order to allow easier customization and easier support for different field types (required for #7)
- WIP Handle unsupported types gracefully (#9)