Faster loading & in-complete "endless" scrolling

- The menu will now always be shown at the top even when scrolling.
- Starting storing the generated html page in localstorage which means
it will now load close to instantanious as it will no longer have to
generate new entries from stored data at every load.
- The addon will now store older entires than those shown on the
notifications page. This is in preperation of making an "endless"
scrolling feature. But for now it will just show all stored notes at
once.
- Improved the way the example image was generated, so hopefully this
will mean less images of the credits and more from the good stuff.