GET /imports should be a valid request

Parent: [EPIC] Extend Media Management Area, improve UI... (aura#390 - closed)

When the dashboard sends a GET request to the /api/v1/imports endpoint tank responds with an empty response along with a 200 status code.

Internally tank requires the showId to be set via a query parameter and the request is therefore invalid. The easiest fix for this would be to send a 400 status code along with the response to indicate that this query parameter is required.

However, it would be nice if tank would actually accept this request as valid and respond with all currently running imports for shows that the user has access to. The showId parameter should only act as a filter. This would be in line with other APIs (specifically in steering) and would also make it easier to monitor state for all imports regardless of the show.