newusers: Only allow slash commands from specific channels

[KockaAdmiralac]

Description

Slash commands can currently be used from any channel on a server. As we don't want people to be using these randomly, we should probably restrict them only to channels which contain information on what these commands do.

Proposed solution

Restrict slash commands for profile reporting-related commands to the reports and staging channels, and display a message when used outside of them.