RocketChat Permissions

This issue is used to track the "Permissions" settings for RocketChat in the "Administration" panel.

Here I only list the items that turn on. You should make sure no other items are turned on except those listed.

## user
* Create Private Channels
* delete-own-message
* Leave Channels
* Leave Private Groups
* Preview Public Channel
* start-discussion
* start-discussion-other-user
* View Public Channel
* View Direct Messages
* View History
* View Outside Room
* View Private Room

In particular, we should double check the following items are turned off.
* Create Personal Access Token: Only authorized roles can use RocketChat APIs. Otherwise people may crawl the chat history and distribute it in other places.
* Mention All: We only allow channel owner to do this. Otherwise it is too noisy.
* Mention Here: Same as "Mention All"