Prepare for a sensible Content Security Policy

[mvz]

If we want to set Content Security Policy in Publify (see https://edgeguides.rubyonrails.org/security.html#content-security-policy-header), some things need to be done:

• Remove inline javascript attributes
• Remove inline styles
• Remove jQuery, since it contains code that sets inline javascript attributes
• Replace <script> tags so it sets nonces