[RSS bot] Admins/mods should be able to unsubscribe any existing feed

[rootkea]

Hello!

RSS bot has the ability to add multiple feeds. But due to #18310 even a normal user can add a RSS feed (imagine nsfw feed) and worst admins/mods can't do anything about it as RSS bot doesn't give any ability to change/delete the existing feeds.

Screenshot here:

That user @rootkea:mozilla.org is a normal user (power 0) but still could add the feed and now the admin can't do anything.

The only option is to mute or kick the RSS bot from the room.

Version information

• Platform: web (in-browser)

For the web app:

• Browser: Firefox 90.0 (64-bit)
• OS: Ubuntu 20.04.2 LTS
• URL: app.element.io