Option to disable e2e-encryption

I would like to have an option to disable all e2e-encryption gui features.

## Background

We develop a custom matrix client that ships in our app. Currently it does not support e2e-encryption. We have planned to support it but its a big task and so it will take some time to get it done.
We also want our customers to be able to use element-web in their browser to have a good desktop experience.
After logging in to element-web the first thing it shows is the setup process for e2e-encryption which is not needed for our customers at the moment.
Additional they can create new rooms which are e2e encrypted which then cannot be displayed in our app.

## Possible solutions

### Server side

Add an option to the file `.well-known/matrix/client` which tells clients to not show e2e-encryption.
Every client could then react to this flag.

### Client side

Add a config option to element-web to disable e2e-encryption.

## Related issues

* https://github.com/matrix-org/synapse/issues/4401

## Pull Request

https://github.com/matrix-org/matrix-react-sdk/pull/5652

Start by reviewing the linked matrix-react-sdk pull request 5652 and the requested .well-known/matrix/client option. Check how element-web currently presents e2e-encryption setup and room creation. Done means the requested configuration can disable the relevant e2e-encryption GUI features for this deployment.

Written by the indexing model from the issue text.