feat(config): new style.nonce option for CSP

[neSpecc]

Problem

If user enables Content Security Policy style-src, editor's styles are not working

Cause

If CSP "style-src" enabled, browser will execute style only if:

• Content-Security-Policy: style-src 'unsafe-inline' — bad solution
• Content-Security-Policy: style-src 'sha256-ozBpjL6dxO8fsS4u6fwG1dFDACYvpNxYeBA6tzR+FY8=' — user have to manually update hash on every editor style change
• Content-Security-Policy: style-src 'nonce-2726c7f26c' — nonce is random number re-generated on each request

Solution

If user enabled CSP, he should generate random nonce on each request, set it in CSP header Content-Security-Policy: style-src 'nonce-2726c7f26c' and pass it to the Editor, so we will add that nonce to the style tag.

Resolves #1334