Disable dump() in Twig when debug = false
Expose Twig functionality more safely in content values

Bolt now exposes a stripped-down context to Twig when used inside a
content value (typically, a HTML body). Features that could potentially
leak internal application state aren't available, and only extensions
that explicitly report themselves as allowed in user content can hook
functions into this restricted Twig context.

Among other things, this means the following are not available inside
user content:

* The `request` object (because it may leak session data and HTTP-only
  cookies into the HTML and JS context)
* The `dump()` function (because it can expose private members of
  various variables, e.g. `app` or `record`)
* The `redirect` function (because it doesn't make sense for
  user-provided content in a Bolt page to cause a redirect)

For detailed information about what is and isn't available, check the
TwigExtension class.
Removed redundant code