Add redirect from 2020-april-workshop

[cosden]

I'm sure there are links all over the place to the old site. It doesn't make sense to have the url be the wrong month, but we also want people to be able to find it via old links.
If anyone knows an easy way to do it, please go for it. Otherwise I'll look into it later.
Just us-rse.org/2020-april-workshop -> us-rse.org/2020-oct-workshop

[cosden]

Actually, maybe it would make sense to rename this to us-rse.org/first-community-workshop. That way if something changes the url doesn't have to again. It wouldn't change that we still need a redirect.

[cosden]

Done in: USRSE/usrse.github.io#184