Session backup (read-only)

[bwindels]

Implement session backup so we can read encrypted history from before we logged in.

• derive AES key from passphrase using pbkdf2 (https://github.com/uhoreg/matrix-doc/blob/ssss/proposals/1946-secure_server-side_storage.md)
  • use https://github.com/brix/crypto-js/blob/develop/src/pbkdf2.js for IE11? or look if we can use https://www.npmjs.com/package/pbkdf2 which can use the native sha512 hashing which should be supported in IE11. Or maybe not.
• decrypt session backup private key in m.megolm_backup.v1 using AES
• implement session backup, see https://github.com/matrix-org/matrix-doc/blob/ce286dac02ca79e505592df58a658a63408497ff/proposals/1219-storing-megolm-keys-serverside.md#auth_data-backup-versions
• prompt to enter passphrase when a session id is missing
• fetch a missing session key from backup once backup is enabled

As we don't have a concept of trust yet, we will only verify that the backup came from a session we trust by the fact that you enter the 4S passphrase, which gives us the session backup key.