Fix oversight in isDraftModified

If the original tiddler doesn’t exist then we should count the draft as
always having been modified
Add support for onhashchange

Now we respond dynamically to changes in the location hash in the URL
bar. It means that you can do links in HTML as `<a
href=“#HelloThere”>go</a>` and in Markdown as `[example
link](#HelloThere).`

We still need to make startup.js more modular