Codebase restructure

[bartolomej]

Description

This significant codebase restructure introduces:

• split frontend/backend apps into multiple apps/modules for easier reuse and better customizability
• greatly improved electron setup (using https://github.com/electron-react-boilerplate/electron-react-boilerplate)
• simplified architecture that allows greater extensibility
• ...

Closes #153

Developed in the scope of maintenance milestone in onflow/developer-grants#150

TODO

• Add back CLI interface (for running with Flow CLI)
• Handle process shutdown (to avoid orphan processes)
• Handle managed account creation
• Add old electron settings (icon, min window dimensions,...)
• Handle gateway configuration
• Fix inconsistent account sorting
• Should the index manage created/updated/deleted dates (then we may need different interfaces for internal/exposed objects)?
• Sometimes emulator doesn't start when using temporary workspace
• Setup prettier/eslint
• Add usage requirement logic back
• Test build on Linux/Windows