Add ability to force disable E2EE via admin config with no user override

### Description

No easy way to disable end-to-end encryption for self-hosted riot instances (I have a legal requirement to make our chat logs auditable)

### Steps to reproduce

- Host a riot instance
- Scour documentation for how to disable E2EE options
- Fail

I expected to find documentation for how to disable E2EE. Instead, I compiled from source after deleting [these lines](https://github.com/matrix-org/matrix-react-sdk/blob/master/src/components/views/rooms/RoomSettings.js#L614-L638).

### Version information

- **Platform**: web

For the web app:

- **Browser**: Firefox
- **OS**: Fedora 29
- **URL**: riot 17.8 self hosted