Refactor to use `render_error_page` to handle User::PrivilegeError
exceptions. This way these exceptions are logged to New Relic.
Changes:
* Anonymous users aren't automatically redirected to the login page.
Instead they're taken to the access denied page, which links to the
login/signup pages.
* JSON/XML error responses return `message` instead of `reason`.
Fixes POST/PUT API requests failing with InvalidAuthenticityToken errors
due to missing CSRF tokens.
CSRF protection is only necessary for cookie-based authentication. For
non-cookie-based authentication we can safely disable it. That is, if
the user is already passing their login + api_key, then we don't need
to additionally verify the request with a CSRF token.
ref: 2e407fa476 (comments)
