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
What I was thinking of was the most important use case to me - being able to do ``u.Quantity(table['somecol'])`` and having it include units if they are in the table.  I'll re-phrase this to reflect that more is coming, though.