message: Use per-message string substitutions in key backup panel
  Including unused substitutions triggers console logs, so change the key backup
  panel to only substitute what's actually used in each message.
  Fixes
diff: @@ -155,46 +155,44 @@ export default class KeyBackupPanel extends React.Component {
  let backupSigStatuses = this.state.backupSigStatus.sigs.map((sig, i) => {
  const deviceName = sig.device.getDisplayName() || sig.device.deviceId;
  - const sigStatusSubstitutions = {
  - validity: sub =>
  + const validity = sub =>
  <span className={sig.valid ? 'mx_KeyBackupPanel_sigValid' : 'mx_KeyBackupPanel_sigInvalid'}>
  {sub}
  - </span>,
  - verify: sub =>
  + </span>;
  + const verify = sub =>
  <span className={sig.device.isVerified() ? 'mx_KeyBackupPanel_deviceVerified' : 'mx_KeyBackupPanel_deviceNotVerified'}>
  {sub}
  - </span>,
  - device: sub => <span className="mx_KeyBackupPanel_deviceName">{deviceName}</span>,
  - };
  + </span>;
  + const device = sub => <span className="mx_KeyBackupPanel_deviceName">{deviceName}</span>;
  let sigStatus;
  if (sig.device.getFingerprint() === MatrixClientPeg.get().getDeviceEd25519Key()) {
  sigStatus = _t(
  "Backup has a <validity>valid</validity> signature from this device",
  - {}, sigStatusSubstitutions,
  + {}, { validity },
  );
  } else if (sig.valid && sig.device.isVerified()) {
  sigStatus = _t(
  "Backup has a <validity>valid</validity> signature from " +
  "<verify>verified</verify> device <device></device>",
  - {}, sigStatusSubstitutions,
  + {}, { validity, verify, device },
  );
  } else if (sig.valid && !sig.device.isVerified()) {
  sigStatus = _t(
  "Backup has a <validity>valid</validity> signature from " +
  "<verify>unverified</verify> device <device></device>",
  - {}, sigStatusSubstitutions,
  + {}, { validity, verify, device },
  );
  } else if (!sig.valid && sig.device.isVerified()) {
  sigStatus = _t(
  "Backup has an <validity>invalid</validity> signature from " +
  "<verify>verified</verify> device <device></device>",
  - {}, sigStatusSubstitutions,
  + {}, { validity, verify, device },
  );
  } else if (!sig.valid && !sig.device.isVerified()) {
  sigStatus = _t(
  "Backup has an <validity>invalid</validity> signature from " +
  "<verify>unverified</verify> device <device></device>",
  - {}, sigStatusSubstitutions,
  + {}, { validity, verify, device },
  );
  }
  