Issue 65: Enable Stream on multisite / network installs

See #154 for additional discussion. Resolves #65 

* [x] Records screen shows mashup of all activity on network
* [x] Remove **Keep Records for** and **Reset Stream Database** from New Site Settings screen
* [x] Private feeds for the network (waiting to hear back on https://github.com/x-team/wp-stream/pull/401#commitcomment-6016479)
* [x] Live updates support in dashboard widget and records screen
* [x] Ensure cron event is fired when **Keep Records for** network setting is changed
* [x] Ensure **Reset Stream Database** action deletes all records (where `type=stream`) for all sites
* [x] Ensure **Keep Records For** setting works on multisite with and without Stream network enabled
* [x] Ensure **Reset Site Settings** action applies the New Site Settings to all blogs
* [ ] Ensure multisite support works with the **Extensions** screen (#396)
* [x] Test DB upgrade routine for those already using Stream on multisite installs
* [x] Move the DB upgrade logic to the #379 branch, and remove from this branch
* [x] Test on WordPress 3.9