demo: Improvements to code presentation

* Display inline template html
* Don't restrict modes to scala/html
* Filename above code
demo: add an import to Boot
web: Multiple ajax pages should not overwrite each other

Two snippets on the same web page can have a different Page.

Also add SimpleAjaxPage#heartbeatInterval configuration
web: Repeater: unused code

Doesn't need to extend HtmlFixer (such work is done in
CanRender[DomMutation]).
Unused imports.
web: AppendToRender: access to current Pages
web: Messages: doc
web: Messages: RequestVar-less default Messages
demo: Switch pages to markdown
demo: CellDemo: limit to 60 seconds

Not 10 minutes
demo: m-dashes, apostrophes, and other formatting
demo: Make the inline code snippets CodeMirror-ed
demo: doc changes (move up "what's the point", header levels)
demo: New documentation layout

Sidebar, inline live demos
Split up EventStream into smaller pages