Handle both themes and plugins email notifications

[audrasjb]

Let's update wp_autoupdates_automatic_updates_complete_notification and wp_autoupdates_send_email_notification functions to handle both plugin and theme email notifications.

Example of an email notification with both themes and plugins updates:

Fixes: #16

[audrasjb]

Example of email notification when only themes were updated:

[pbiron]

LGTM. I like the idea of only sending 1 email for both kinds of updates. Avoids email overload and makes it more likely that admins will actually read them.

[audrasjb]

Great. Let's merge this one 👍