Allow me to report a message to the room's admins

**Is your feature request related to a problem? Please describe.**
I recently learned that if I report a message, the report goes to my instance admin¹, not the room's admin. The admin of my instance can't do anything about an user on a different instance (AFAIK) and if they could, they are not qualified to enforce the room's rules.

**Describe the solution you'd like**
I want to be able to notify the admin of the room about a message that is against the room's rules. An option to DM users (or just admins) in the member list would be a good start. An option to report a message to the admins directly would be better.

**Describe alternatives you've considered**
This is how I worked around the limitations:
* Write down ID of admin.
* Start a private conversation with the admin.
* Copy the permalink of the message.
* Paste the permalink and point out what about it is against the rules.

That's an awful workflow, especially on a “phone”.

**Edit**:
¹ The information I got was: “It gets sent to your own homeserver, and with synapse it just goes in a database table *somewhere*”

Here is the relevant part of the spec:

> Servers are free to handle the reported content however they desire. This may be a dedicated room to alert server administrators to the reported content or some other mechanism for notifying the appropriate people.