Display images embedded into DMs

Accessing images embedded into DMs requires proper OAuth token.

There is no way to tweak headers for an image request from QML - so we instead implement an image provider (subclass of `QQuickImageProvider`) that can pass custom headers alongside the request it sends.

Implemented image provider captures all `http://dm/<auth-token>` requests and then requests images passing the given auth-token as `Authorization` header. Image URL itself is extracted from the `realm` parameter of the auth token.

The rest of the changes are simple plumbing passing media url from API into the database and into the model.