Migrate SSSS to symmetric

[uhoreg]

Depends on matrix-org/matrix-js-sdk#1238

Scenarios:

• user has existing asymmetric SSSS, user logs in to Riot
  • works, but user has to enter SSSS passphrase twice for some reason
• user has existing asymmetric SSSS, user reloads already-logged-in Riot
  • works, but upgrade encryption toast doesn't seem to go away after completing the upgrade
• user has existing key backup but no SSSS, user logs in to Riot.
• user has existing key backup but no SSSS, user reloads already-logged-in Riot
• user has no backup/SSSS, user logs in to Riot
• user has no backup/SSSS, user reloads already-logged-in Riot
• user has symmetric SSSS, user logs in to Riot
• user has symmetric SSSS, user reloads already-logged-in Riot

For migrating existing key backup, no SSSS, react-sdk needs to pass the backup recovery key to js-sdk somehow. This would be done in the the _bootstrapSecretStorage function of CreateSecretStorageDialog.js. This would either be done by prompting the user and then passing the key to js-sdk's bootstrapSecretStorage (if CreateSecretStorageDialog knows when it needs to prompt for the recovery key), or by passing a callback to js-sdk's bootstrapSecretStorage, which will be called when bootstrapSecretStorage needs the key. js-sdk currently hardcodes a random key at https://github.com/matrix-org/matrix-js-sdk/pull/1238/files#diff-fa4472db829c5a59596c29a2f43e8f9cR506 .