Tidying up genstat changes

This _looks like_ a massive PR, but it's actually not so bad. There are a couple of minor additions:

* Running notes 'Add note' form is now a the top of the list, rather than the bottom
* New button to hide the pre-production dates from the timeline at the top
* Some minor reformatting of tables and other bits and pieces

Main changes are:

* Deleting the old dead files (`base.html`, `status.css` and some unused template files)
* Renaming `base_new.html` to `base.html` and updating all of the links
* Restructuring `status.css`
  * I grepped any styles that I didn't recognise, and deleted them if they weren't used
  * I restructured the file into four main sections
    * Each section has a title and description, with subheadings
  * I wrote an introduction with some very basic style guides.
  * Where there were styles with unclear use (many that I had written in the last week or two) I tried to add some comments to say what they do.

So in short, this has already been reviewed in the previous PRs except for code structure updates.