NSFW rooms only ban - web admin

[Bluestrings-Drawpile]

From #914 ... Bans, 4 Priority: Medium

Ban users only from rooms marked NSFW option, so they can access SFW content on the server.

Problem:

• Suggested by Liz, the intent was so a server owner wouldn't have to set up two servers to achieve this.
• There's also the consideration for servers that allow mixed content of SFW and NSFW content.