Show a what's new in this version popup

[julianjelfs]

Seems like there are a few things to nail down on when and how this would show.

Presumably it should not show for a new user i.e. they must have seen the previous version to be interested in what's new in this version.

We only want to show it to them once.

Possibly we also might want a page that lists what went into all versions that people can go to (and the popup just shows the most recent entry).

Etc etc.