WIP: online incremental megolm backups

[ara4n]

This doesn't remotely work yet (not even tried to compile it yet) and isn't finished, but opening a PR to aid feedback/review

Listens to new crypto.suggestKeyRestore event, and if it fires shows a new SuggestKeyRestoreDialog to prompt the user to restore their old keys; either by restoring from online backup, importing offline backup, or verifying with a device such that the device syncs their keys over. It also adds a SuggestKeyBackupDialog to prompt the user to backup their old keys.

Stuff left to do includes:

• Encrypting and saving new inbound sessions to the server (i.e. turning the comments into code)
• Implementing onRecoverFromBackup (turning comments into code)
• Implementing onStartNewBackup (generate backup key; give it to the user; encrypt all the keys and upload them)
• Hooking up the various checks on backup state to js-sdk (and thus Olm)
• Hooking up online backups so you can enable/disable them from UserSettings

Later on:

• Proactively sharing all sessions when you verify your own device
• Cross-sign and publish on /devices the devices you've verified yourself, both so people can trust you (or distrust you if you revoke), and also so we know whether to prompt to restore keys or not!
• Implementing better verification so users just have to compare a phrase (e.g. uhoreg's https://cryptpad.fr/pad/#/1/edit/0eqaXl2wZhiq5s5Ty-wzcg/QL4+6V9poZCmbZI0ispNMFbU/)

This is twinned with...

• WIP: online incremental megolm backups matrix-js-sdk#595
• https://github.com/matrix-org/olm-backup/pull/47
• Initial APIs for storing E2E room keys synapse#2731

[dbkr]

Superseded by #2169