Add symmetric encryption

[vertigo220]

I'd really like to have the ability, as some other services do, of sending an encrypted message to users of other services via symmetric encryption. This works by, instead of sending the message, saving it encrypted on Runbox's servers and sending the recipient a link to access it. When they click the link, they're asked to provide a password, which would be shared another way, whether by phone, text, in person, etc. Once they enter the correct password, they would have access to the message as well as the ability to reply, which would be sent back to the Runbox user, encrypted (obviously, it could go straight to their inbox, since it would all happen on Runbox's servers).

There should also be some options the Runbox user can use associated with this, aside from, obviously, the password. First would be how many attempts the recipient can make to enter the correct password before the message is permanently locked and deleted. Second is how long the message should remain before the same is done, i.e. an expiration. And it should allow attachments, so files can be sent encrypted as well.

[gtandersen]

See also #1462.

[gtandersen]

To clarify: This would utilize a single key to encrypt and decrypt, in contrast with asymmetric encryption as e.g. PGP.