Ability to disable End-To-End Encryption via Config

This issue has been migrated from [#4401](https://github.com/matrix-org/synapse/issues/4401).

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**Description:**
Originally proposed in vector-im/element-web#4367 - for the case of vector-im/element-web#4367 is was sufficient to address via power level settings. Creating a new issue to track doing this via a config setting.

The original ask:-
"I would like the ability to disable end-to-end encryption in my self hosted Synapse instance. I have a legal requirement to provide audit-able chat logs, which is obviously impossible if there's nothing preventing my end-users from encrypting any room they create."