Filter wp_mail() on .org from trying to mail prohibited addresses

[Ipstenu]

We have certain email providers (eg Yahoo) prohibited from signing up for new user accounts, due to bounce back issues.

We should prevent wp_mail() from even trying to email them, and reduce load (and lost emails) on our end.

[dd32]

In 9242:

Support: Disable entire-forum subscriptions.

Some uses have managed to subscribe to entire forums, it's probably not intended, so lets ignore these for now.

See #4782.

[dd32]

I've added something in the private repo that does this:

r15456-dotorg:

Mail: Add an mu-plugin to filter outgoing wp_mail()'s.

This blocks emails to hosts we consider "unsafe" which is a bad term, but is roughly equal to hostnames we consider spam or who we have mail deliverability issues to.

A PHP warning will be triggered when this happens for logging / discoverability purposes.
It'd be nice to add a `Require-Recipient-Valid-Since:` header for appropriate hosts once we start to store user email change dates.

See https://meta.trac.wordpress.org/ticket/4782

In addition to #4781 we should also prevent new forum subscriptions for affected users.

[dd32]

In 9301:

Support: Disable entire-forum subscriptions (again).

bbPress 2.6 deprecated the previous function and no longer calls its filters.
This more complicated filter should hopefully have the same effect again.

See #4782.

[dd32]

In 9302:

Support: Remove the Subscribe-to-forum links.

See #4782.