Display images inline in Direct Messages

Accessing images embedded into DMs requires proper OAuth token.

There is no way to tweak headers for a message from QML - so we instead
implement a QImageProvider that can pass custom headers alongside the
request it sends.

Implemented image provide captures all http://dm/<auth-token> requests
and then requests images passing the given auth-token as
`Authorization` header.

The rest of the changes are simple plumbing passing media url from API
into the database and into the model.