SVG 2 Editing: Marking changes

As agreed in the last meeting, PLH & Chris published an updated CR for SVG
2 this week: https://www.w3.org/TR/SVG2/Overview.html

For keeping track of changes, I've added a new class that changes the
"Changed since..." annotation to have the more recent date, so that it is
easier to review later.  To minimize merge conflicts, I kept the old class
name the same, so the new markup would be:

```
<div class="changed-since-cr1 cr2">
  <ul>
    <li>Fixed that terribly broken thing.</li>
  </ul>
</div>
```

(They are still changes since CR1, they're just *also* changes since CR2! I
hope that makes sense, without needing too much typing...)

When making these changes, I discovered that Nikos' initial plan was to use
the "changed-since-cr1" class in the actual spec text (not just the changes
appendix).  But no one had been doing that during editing this year, so I
removed the only two cases where it existed. With lots of changes being
deletions, highlighting changed sections wouldn't be terribly effective,
anyway. The changes appendix allows for more details & links to issue
discussion.


In summary:

- Always make sure there's a changes appendix entry for any PR you make or
review.

- For anything merged this week or later, make sure the change is grouped
in a `<div class="changed-since-cr1 cr2">`

- Add a link to the issue discussion that led to the change. (As I'm
reviewing the changes list, I'm going to do this for the changes since CR1).

~Amelia