Add option to encrypt keystore

[kkuehlz]

Like a password manager, give users the option to have the keystore encrypted at rest. This means that, for every new browser session, the user will need to enter their decryption password before it can be used. We can put a nice lock UI on the popup.

[dkess]

What threat model is this supposed to cover?

[yvbbrjdr]

Keystore Syncing