Plugin Directory: Don't send rejection emails to blocked users

[Ipstenu]

In order to reduce ongoing abuse and harassment by misbehaving developers, as well as prevent totally unnecessary email being sent, we should stop emailing blocked devs when their plugin is rejected..

This will effectively not notify them that their plugin has been rejected, but will still allow us to email them via the plugin admin page should we deem it necessary.

[Ipstenu]

Don't email blocked users on plugin rejection.

[dd32]

In 10124:

Plugin Directory: Convert Plugin Approved/Rejected emails over to the new Email class.

Additionally, this causes rejected emails not to be sent to blocked users and updates the text slightly.
Props Ipstenu.

See #5351.
Fixes #5354.

[dd32]

Hopefully this is obvious enough, the audit log entry is Plugin rejected. / Plugin rejected. Email not sent..

The main reason for an email not being sent would be if the user is blocked, but could also be for other reasons (other filtering, etc)