Room permission for creating "top-level" messages; those without it can only thread reply

**Describe the solution you'd like**
A room permission that makes it so only certain members can create top-level messages in the room, and those without it are only allowed to reply via threads (or a reaction). This would allow a forced thread structure for rooms focusing on highlighting specific content, and allowing only replies to that content.

**Describe alternatives you've considered**
This can kind of be enforced with bots but not as cleanly.

The issue names no files, tests, or entry points. First identify which Element component owns room permissions and message creation, then confirm that members without the new permission can only thread-reply or react, while permitted members can create top-level messages.

Written by the indexing model from the issue text.