Handle themes auto-updates

[jeffpaul]

Migrated from: audrasjb/wp-autoupdates#65
Previously opened by: @jeffpaul
Original description:

If we're going to track plugins auto-updates, we should similarly track themes auto-updates. Noting that this issue may lag the plugins work a bit given the differences between how themes and plugins are maintained on WordPress.org.

[bookdude13]

The start of this was handled in:
audrasjb/wp-autoupdates#44 (basis)
audrasjb/wp-autoupdates#78 (themes.php single site)
audrasjb/wp-autoupdates#82 (update-core.php)

[audrasjb]

Fixed by:

• Add themes autoupdates support basis audrasjb/wp-autoupdates#44 (basis)
• Further basis for themes auto-updates audrasjb/wp-autoupdates#78 (themes.php single site)
• Add Themes auto-updates to update-core.php screen audrasjb/wp-autoupdates#82 (update-core.php)
• Avoid duplicate Updating... dialog #32
• Add and populate Automatic updates column, add and handle enable/disable auto-updates bulk actions to the multisite themes list table. #33
• Handle both themes and plugins email notifications #36