* src/client/components/client-web-view.vala (ClientWebView): Register
  for new remoteImageLoadBlocked message from JS. Add new
  ::remote_image_load_blocked signal to notify when a remote image load
  was blocked. Add ::allow_remote_image_loading and ::load_remote_images
  methods to allow th app to manage image loading state.

* src/client/conversation-viewer/conversation-email.vala
  (ConversationEmail): Determine up front whether the message view should
  load remote images and flag that, rather than passing through the email
  flag and the contact store.

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage): Rmeove contact_store and
  always_load_remote_images properties and related code, just let the
  parent email view advise whether remote images should be initially
  loaded and act accordingly. Look up to ClientWebView signal and methods
  to handle manual remote image loading management by the user. Remove
  some obsolete code.

* src/client/web-process/web-process-extension.vala (GearyWebExtension):
  Replace allow_prefix implementation for remote image management with
  explicit signalling to/from the application via JavaScript.

* ui/client-web-view.js: Add a new PageState object, add props and method
  to implement remote image loading management.

* ui/client-web-view-allow-remote-images.js: Monkeypatch to be used when
  remote images should be loaded by default.

* ui/CMakeLists.txt: Include new JS file.

* src/client/conversation-viewer/conversation-web-view.vala,
  src/client/web-process/util-conversation.vala,
  src/client/web-process/util-webkit.vala: Remove obsolete code.