Restrict dashboard.js to only enqueue on Dashboard (index.php)

This resolves an issue where dashboard.js was being included on every
page, and the Heartbeat Ajax response would include a PHP notice:

> Notice: Undefined index: wp-stream-heartbeat-last-id in .../stream/includes/admin.php on line 792
Merge pull request #410 from x-team/issue-409

Issue 409 Separate dashboard / ajax actions from admin.php.
Merge pull request #408 from x-team/restrict-dashboard-js-enqueue

Restrict dashboard.js to only enqueue on Dashboard (index.php)
Fix pagination bug in Dashboard widget caused by total record count
Format the current page number in Dashboard widget pagination controls
Disable Dashboard widget next/last pagination controls on last page
Merge branch 'develop' into issue-65

Conflicts:
	includes/admin.php