Split org/people page into current and past

Closes #697, but with these caveats:

- The duplicate people in the list (see issue) has not been addressed, as the page is actually listing positions. Changing it to be a list of people (which is what it does look like) would need deeper changes.
- The way the query parameter `historic` is added to url will clobber other query parameters. Not an issue atm though.