DOC: Redirect old whatsnew links
In https://github.com/pandas-dev/pandas/pull/21599 we're chagning the structure of our whatsnew. Previously the whatsnew for each version was concatenated into a single large file.

Consider a link to the anchor `#whatsnew-0190-enhancements-other`. There are three cases:

1. Version-specific URLs like http://pandas.pydata.org/pandas-docs/version/0.23.0/whatsnew.html#whatsnew-0190-enhancements-other will continue to work. We don't have to change anything.
2. People linking via `/stable` (which is probably more common), or to version 0.24.0 or higher *will* break. http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#whatsnew-0190-enhancements-other won't be a valid reference. We would need to rewrite it to http://pandas.pydata.org/pandas-docs/stable/releases/#whatsnew-0190-enhancements-other

We use nginx for our webserver at pandas.pydata.org. But AFAICT, we can't use a rewrite rule to redirect this links, since nginx doesn't see the anchor.

We should be able to detect this in Javascript. When we're on the whatsnew for 0.24.0 or greater, we want to look for anchors pointing to `/whatsnew.html#whatsnew-<version>` for 0.23.4 or older. Then we'd rewrite `/whatsnew.html#whatsnew-<version>-<title>` to `/whatsnew/<version>.html#<title>`. So a test case is


```
before: http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#whatsnew-0190-enhancements-other
after:  http://pandas.pydata.org/pandas-docs/stable/whatsnew/v0.19.0.html#other-enhancements
```