Load remote attachment URLs and provide the user the option to load it

This matches current Mastodon Web behavior with blocked media, where the attachment won't be loaded in the client but the user has the open to open it.

Ideally, we should present the option to load the media in client - but Mastodon doesn't give us remote attachment information easily. We would have to glean information from the URL, contact remote servers or other complex nonsense. Right now, this just opens blocked media in your web browser.

Also without this MR, Tokodon would follow the broken media link (since it's blocked, Mastodon returns /files/missing.png which isn't actually a valid image funnily enough) and Qt would scream at us for the audacity of giving it a missing image. This should quiet down the log a bit since it won't load the image unless it's actually AttachmentType.Image.