Allow bulk-adding users to an org from the "users not in an org" list in the root

[stuartlangridge]

1. hit some sort of "add users to an org" button on that front-page "users not in an org" widget
2. all user rows get a checkbox
3. check all the ones you want to add
4. press some sort of "ok add them" button
5. choose an org (or create a new one)

This doesn't make it easy to add users already in org A to org B, but that's not a huge problem, and it is a lot easier than going to each org member's contributor page.

Obvious issue with this: dates are a problem because they're different for each user. @jeremy-lq perhaps you have thoughts on alternative ways to do this that are easier than visiting each contributor page in turn?

[stuartlangridge]

Maybe a "manage org memberships" page would be better than trying to build this into existing widgets (or at least would be a good addition to those).