Support only E2E or public rooms on my homeserver

This issue has been migrated from [#3269](https://github.com/matrix-org/synapse/issues/3269).

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**User story:** I am a well-intentioned sysadmin. I want to host a homeserver on the Internet, so that I can help strangers communicate without myself being coerced into reporting user activity.

**Suggested solution:** Configure the homeserver so that only two room types are supported:

1. E2E rooms with limited access. Only end-users can see the contents of the room.
2. Cleartext rooms with public access. The conversation is stored in cleartext on my server, and anyone with the room URL can read the conversation.

Thus, cleartext rooms with limited access cannot be created or accessed *from this homeserver*. Users who need such rooms can use a different homeserver.

**Rationale:** https://code.puri.sm/purist/services/wiki/pattern_trustworthy_service