Adding Media to objects

This MR adds media to objects through the following routes:

• GET /api/:object_type/:object_id/media -> List all media of the object
• GET /api/:object_type/:object_id/media/:medium_id -> Get one media item
• PATCH /api/:object_type/:object_id/media/:medium_id -> Update one media item attributes
• PUT /api/:object_type/:object_id/media/:medium_id -> Update the item uploaded (reuploads)
• DELETE /api/:object_type/:object_id/media/:medium_id -> Destroy a medium

valid attributes:

• title STRING
• alt_text STRING
• item FILE

body:

{
  "medium": {
    "title": "blabla",
    "alt_text": "blibli"
  },
  item: File
}