What's new for 0.4 updates

This PR adds to and polishes the what's new page, based more-or-less on some discussion from #2416 .

It adds a new `Quantity` section (@mhvk, you might want to glance over that), adds a variety of cleanup/missing stuff, and re-works the introduction to include some numbers about how many issues, PRs, and contributors were involved in this version.

For that last step, I used https://gist.github.com/eteq/9faf252ec8aa41188dcb, which uses the github api to search through all the issues and PRs closed since v0.3.  That's worth noting for future versions, as it probably makes sense to reproduce this there.

cc @astrofrog @embray
@eteq - `Quantity` does not yet interact all that well with `Table`, so that should be removed.  I think it would be good to state that "..., while work is continuing to integrate with `~astropy.table` as well".