• Find somewhere in indexeddb for the USK and SSK private parts to live (suggest maybe the 'account' table with keys of ssss_cache:[key name]? Or make a separate 'keys' store but that will require another db version bump which will break compatibility).
• Store the private keys in in Olm pickle format so in the future we can have an encryption key for all crypto data store in indexeddb (all the other sensitive data is Olm pickles which are encrypted).
• When we require the SSK and USK, check for them in indexeddb, then if we don't find them, continue & prompt for SSSS master password as previously. This should also cover when the user enters their passphrase at login time, so if they do this then they'll have their USK & SSK cached.
• Once the user successfully enters the passphrase, extract USK & SSK and store them in indexeddb.

Requires #12301