Add interface for third-party apps to provide images for email addresses

Right now we only display images in the message list when the sender's email address belongs to a system contact. Sometimes it would be nice to see pictures for the sender without having to add them to you contacts.

Examples:

• GitHub's notification emails contain enough information to be able to fetch the avatar of the person who triggered the notification
• Gravatar (allows finding an avatar using an email address)
• Favicon (see #6128)

There's a lot of sources one could use. I don't want to include support for any specific provider in the app directly. Instead we should add an interface that allows third-party apps to provide an image given an email address and/or a specific email header field.

The issue names no files or tests; start by reviewing the existing message-list handling for system-contact images. Define the extension boundary for third-party apps and how an email address or specific email header is supplied. Done means third-party providers can supply sender images without adding a provider directly to the app.