Implement permissions for individual threads and forum posts

Individual threads or forum posts cannot have permissions assigned specific to them, instead inheriting the permissions of the parent channel. The only per-thread options currently are Hide After Inactivity and Slowmode.

Allow each thread/post able to have its own permissions set. For example, a showcase channel where only the OP of a forum can send messages, or one where the OP and/or designated roles have the ability to edit the thread, pin messages, manage reactions, or delete any message within the thread. A forum post or thread's permissions would be able to changed by right clicking, selecting Edit Thread/Post (depending on the type), and configuring a list of permissions for that thread or post, identical to how a typical channel's permissions would be configured.

Post #4016 suggests something similar, assigning permissions automatically through the use of tags. In addition to this post, if automatic permissions with tags are added, the exact permissions could be controlled through the same method as the channel itself.