fix: Use preload script as proxy for main-renderer communication

[amaury1093]

fixes #497

Also:

• Remove isParityRunning everywhere, as we're not using it at all
• Don't store language preferences in localStorage, fixes Don't store language preferences both in electron-settings and localStorage #509
• Tightened CSP

All messages between main and renderer have the following format:

{
  action: string, // in capital letters, ending in _REQUEST or _RESPONSE
  from: 'fether:electron' | 'fether:react',
  payload: any // Additional payload to the message
}

[amaury1093]

I tested (in both yarn electron and yarn start):

• closing/re-opening a node in a terminal
• changing language

[amaury1093]

If someone has an idea how not to hardcode these origins, I'm all ears

[amaury1093]

prefetch-src gave me a "unknown csp" warning, so I removed it

[amaury1093]

@yjkimjunior I needed to add 'self', or else yarn start didn't work

[Tbaut]

I could send a Tx on Kovan without problem. Switched language as well.

[axelchalon]

👍