Support Forums: Introduce direct moderator replies

[Clorith]

See #5717 for initial reference about "anonymous" notifications from moderators on topic reports. There's a case to be made for adding the same kind of notification system when needing to address individual users for guideline violations.

This would prevent moderators from accidentally "shaming" the user for a moment publicly, while their reply is visible, and also letting everyone know who acted on that one topic. Unfortunately that's the state of the internet today, we need some more tools to protect our volunteers at this level.

Much like the proposal for a custom comment type for reports, the same could be applied here, a custom comment type for moderator replies, having this as a toggle, that's always off by default, but checked when doing a special post, which would hide the topic instantly from any non-moderator users, but would send notifications as usual to any user mentioned.

These notifications should then also be sent anonymously, just from a generic "WordPress.org Forums" account, perhaps with a fixed text appended, along the lines of:

This notification is in relation to <topic title> (<topic url>), if you have a question or concern relating to this, please see the #forums channel on Slack (​https://make.wordpress.org/chat) and let the team know which topic you are seeking clarification on.

[sterndata]

There needs to be a way for other mods to see what one mod has sent, lest we start swarming.