Issue 356 Add live updates to dashboard widget

Live Updates for Dashboard Widget.

* [x] "Enable live updates" checkbox
* [x] Default setting to true on install
* [x] Configure heartbeat response
* [x] One method to fire on `heartbeat_recieved` to handle Dashboard and Stream Page requests (DRY)
* [x] One method to generate list items for dashboard widget, whether initial page load or heartbeat
* [x] JS to handle updating the Dashboard
* [x] When new items are added, remove correct number to keep `records_per_page`
* [x] Only display updates on first page of results
* [x] Highlighting/Fadeout of new items

Fixes https://github.com/x-team/wp-stream/issues/356

@fjarrett