* SecureBackupPanel: replace `isKeyBackupTrusted`
`MatrixClient.isKeyBackupTrusted` -> `CryptoApi.isKeyBackupTrusted`
* SecureBackupPanel: replace `getKeyBackupEnabled`
`MatrixClient.getKeyBackupEnabled` -> `CryptoApi.getActiveSessionBackupVersion`
* SecureBackupPanel: replace `deleteKeyBackupVersion`
`MatrixClient.deleteKeyBackupVersion` -> `CryptoApi.deleteKeyBackupVersion`
* Do not show session count if we have no info
We shouldn't say "zero sessions to back up" if we don't know.
* SecureBackupPanel: distinguish between server and active backup
* SecureBackupPanel: stop calling `checkKeyBackup`
`checkKeyBackup` will start key backups if they aren't already running. In my
not-so-humble opinion, the mere act of opening a settings panel shouldn't change anything.
* fix SecurityUserSettingsTab test
`MatrixClient.crypto` is going away, so let's switch over to `getCrypto()`.
There doesn't seem to be anything else relying on the `crypto` stub in
`mockClientMethodsCrypto()`, so let's get rid of that.
* Settings panels: avoid exceptions with rust crypto
If we are using rust crypto, `client.crypto` is undefined. We'll need to fix
these up better in future, but for now, just return early.
* Update tests
