Fixes#1083
Directives are rendered as templates and their bodies are MessageText instances.
We thereby achieve the necessary nesting of directives (and other rich
elements inside directives) by letting each directive
body render itself similarly to how the whole message body is rendered.
Changes how rich message markup is generated and kept track of.
We now have a MessageText class which subclasses String and which keeps
track of the rich markup associated with the message.
Also, show topic by creating a message object, instead of directly
inserting HTML into the DOM. This is a necessary precursor to being able
to render chat messages via lit-html/lit-element.
