* ui/conversation-web-view.js: New script, port old HTML cleaning code in
vala to Javascript as new subclass of PageState. Instantiate that on
page load.
* src/client/conversation-viewer/conversation-web-view.vala
(ConversationWebView): Load and add new JS script for conversations.
* src/client/web-process/util-conversation.vala (Util.Conversation):
Remove migrated and obsolete code.
* ui/client-web-view.js (PageState): Allow on-load behaviour to be
overridden in subclasses.
* ui/CMakeLists.txt: Include new JS script.
* ui/conversation-web-view.css: Chase CSS class name changes.
