Since we're no longer using the web view to display the user avatar, use
libsoup and add some additional infrastructure to support caching the
avatars. Also switches to HTTPS for accessing the Gravatar service.

* src/client/application/geary-application.vala
  (GearyApplication::get_user_cache_directory): New method to return the
  XDG cache directory for Geary.

* src/client/application/geary-controller.vala: Add both a Soup session
  and cache for fetching avatars. Write the cache to disk on controller
  close.

* src/client/conversation-viewer/conversation-email.vala
  (ConversationEmail::start_loading): Trigger avatar loads when loading
  the email.

* src/client/conversation-viewer/conversation-message.vala: Replace
  single avatar image widget with two, so the image does not need to be
  rescaled when expanded/collapsed.

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage::load_avatar): Queue a request for a Gravatar
  avatar.
  (ConversationMessage::set_avatar): Load pixbuf returned by Gravatar,
  scale and set it for the preview and expanded avatar images.

* src/client/conversation-viewer/conversation-viewer.vala
  (ConversationViewer::clear): Cancel any outsanding avatar loads.

* src/client/util/util-gravatar.vala (Gravatar): Construct a HTTPS URL to
  avoid advertising to the NSA who we are receiving email from.

* ui/conversation-message.ui: Add the second avatar image.