Add encryption (#642)

* Add encryption

In order to avoid the server being able to read the content of the scene, this PR implements local encryption and decryption. This implements the algorithm described in #610.

Right now the server doesn't support uploading binary files. I mocked the server with comments. @lipis, could you add support on the server and update this PR? I added a bunch of TODO: that tell you where to comment/uncomment in order to get the server flow going.

To test locally right now:
- Import: Open http://localhost:3000/#json=1234,5oYVOnGpWYPPTz19-PMYYw and see a square
- Export: Click the export link and see the right url with the private key + the encrypted binary in the console

Fixes #610

* backend_v2

* v2

src/scene/types.ts

@@ -18,6 +18,7 @@ export interface Scene {
 
 export interface PreviousScene {
   id: string;
+  k?: string;
   timestamp: number;
 }