Ensure webpack compiles in downstream projects

[aphelionz]

This PR removes leveldown as a direct dependency, instead relying on the implementation of level in orbit-db-storage-adapter, which in turn handles browser vs node dependency switching.

In addition, it utilizes a neat trick that uses eval to require the fs module, ensuring webpack compilation while still allowing node to use fs

[shamb0t]

Not sure I understand the need to use the eval trick. Shouldn't this line

orbit-db/conf/webpack.config.js

Line 16 in 9b58f42

fs: '{}',

allow webpack to compile?

[shamb0t]

Ah got it for other projects to compile using webpack