* ui/conversation-web-view.css: Prefix the names of base Geary-internal
  elements with "geary_" to reduce the odds of them colliding with class
  names in HTML messages. Chase the name changes in classes that generate
  them.

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage): Use constants for class names and
  WebKit.DOMElement::class_list to add/remove them.