Element-R: add labs button to migrate to rust crypto

[richvdh]

The part of migration to Rust Crypto that actually adds a labs button. Not a strict dependency, but we probably don't want to land this until matrix-org/matrix-js-sdk#3964 is done, since otherwise it will eat peoples' data.

We make the existing "Rust cryptography implementation" labs button turn-on-able. (Currently, it is always disabled; change it to only be disabled when the feature is on; see RustCryptoSdkController.) We also make it reload the app when it is switched on; see ReloadOnChangeController for examples of how to do this. Some changes needed to the wording on the button.

This means that, once we restart, MatrixClientPeg.initClientCrypto will find useRustCrypto is true, causing it to call MatrixClient.initRustCrypto instead of MatrixClient.initCrypto. (This will be much the same codepath as when we drop legacy crypto support, which is good.)