refactor: Simplify the jupyter plugin

[DanielSchiavini]

This reverts commit 50ee8e93

What I did

• Refactored a bit the code now that we have the tests in master

How I did it

• By removing proxy functions from javascript and calling the rpc directly from python
• Moved the BrowserRPC up in the file and injected it into the BrowserSigner so all calls go through it

How to verify it

• Plugin should keep on working

Description for the changelog

N/A

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