Backend v0

[zomglings]

@Andrei-Dolgolev : Coordinage with @peersky to understand what endpoints he needs on the backend.

Coordinate with @kompotkot to understand how to read data from blockchain database.

Coordinate with @Yhtiyar to understand how to expose smart contract decompilation metadata to @peersky on the frontend.

[zomglings]

@kompotkot : I suggest to create a db/ folder in which we define models and alembic migrations for the blockchain database.

Folder structure:

moonstream
|-- frontend
|-- backend
|-- db
|-- crawlers
    |-- ethereum
    |-- etherscan

[Andrei-Dolgolev]

Add subscription limit on create subscriptions