Plugin Directory: Update rejection email to be more clear about what happens next

[Ipstenu]

In order to reduce emails of "But why..." and to cover the case where someone resubmitted with a separate account and we forgot to reject the first one, I've tweaked the email.

Now included:

• Plugin permalink
• Original submission time

This also includes updating the last_updated post meta to now, which SHOULD resolve the issue of old rejected plugins not showing up as rejected this week.

[dd32]

In 10023:

Plugin Directory: Update the rejection email to be more clear about what happens next.

Props Ipstenu.
Fixes #5299.