ENH: Add "Creator of a thread" as a "virtual role"

In some case it can be useful to have a forum where user can create a topic but is only able to see it's own topics.

## Describe the solution you'd like
Add a virtual role "Topic Creator" to the permissions grid.
This way you can add a separate forum where a user can only see their own posts.

## Describe alternatives you've considered
Have a separate checkbox for this option, but it seems counter intuitive to me to have "view" rights in two diferent locations

## Additional context

Start by locating the permissions grid and the existing virtual-role handling in the DNN Community Forums codebase. Trace how forum visibility and topic ownership are evaluated, then determine where a "Topic Creator" role would fit. Done means the new virtual role appears in the permissions grid and allows users to see only topics they created.

Written by the indexing model from the issue text.