Implement nip17 DM handling with user-initiated decryption

[aybanda]

 This PR implements handling for nip17 direct messages in the chat interface. 
 Users are prompted for decryption when receiving encrypted messages. 
 The feature includes:
 - User-initiated decryption for nip17 DMs
 - Display of decrypted messages in the chat interface

[Giszmo]

The way this is now, I get bombarded with at least 50 alert windows to confirm something when opening a chat with less than 50 messages.

I suspect @hzrd149 sees a problem with nip17 integration in that prior to decryption, it is not clear who's message we are decrypting as the sender is anonymous at that point.

But as there is absolutely no way of knowing not even the timestamp of the message as per nip17 that is also supposed to be fuzzy, the user will get prompted to decrypt all messages from friends or foes alike? That's not practical.

For what it's worth, I forbade the popup to appear again and clicked on decrypt-all above, which gave me no nip17 messages although there should be some on that friend's chat. I had not analyzed the code to that detail on whither these nip17 messages should be intermixed with nip4 but I suppose that's how it should work eventually.

[Giszmo]

I see nothing in the code that would allow authoring nip17 events. I guess this is still a draft?