Send messages as unidentified sender

[gferon]

TODO:

• Store profile keys somewhere

[gferon]

@Schmiddiii would you mind giving this a test run? Just make sure to set unidentified to true when calling Manager::send_message.

[Schmiddiii]

• 1-to-1 seems to work fine (I sent a message to you).
• note-to-self errors with "Websocket closing while sending request"
• Groups do not yet have unidentified sending?
• When should a client use unidentified sending and when not? Should this maybe be determined inside presage without that public unidentified-parameter?

[gferon]

@Schmiddiii this was WIP, and all of your concerns are now addressed. I'll go ahead and merge this right away 🚀