Add support for userless (System) forum replies, bang commands.

[MauAbata]

Forum threads should have support added for system messages, such as Locked/Unlocked, Merged, etc. posts.

And, for the time being, bang-commands should be parsed out of admin/moderator posts and handled in an after-save async job on the forum post. Workflow:

1. New Forum reply is created,
2. After commit, scan for /^!\w+/ lines and if any are found, schedule a Forums::CommandProcessorJob to run on that post.

In the Command Processor Job:

1. Scan each line for any matching bang commands and their arguments,
2. Validate permissions for each using Pundit, and take the appropriate action,
3. Log a system post to the thread regarding that action taken.

For the System Posts:

Post content is a serialized collection of data required to render the notification, which includes a proper I18N key for the message to be displayed and placeholders to fill in that message. Or something.

Usecase:

!merge <url>
!lock
!unlock
!makesticky
!clearsticky
!flagdelete
!move <forum>
!resolve