Restrict paired browsing to when dev mode is enabled - #3863

Summary

See https://github.com/ampproject/amp-wp/pull/3656/files#r352310999:

I just realized that this makes every page invalid AMP, because every page is forced to be in dev mode and the custom script is enqueued. Two options I see:

1. Short-circuit this function if an user is not logged-in (or rather, if amp_is_dev_mode() returns false). Since search crawlers will never be logged-in, they will never then see an invalid AMP page. This would also require entire paired browsing app should wp_die() if the user is not logged in.
2. Go back to only including this JS if the admin bar is showing. Accessing paired browsing here would still need to include the wp_die() if ! amp_is_dev_mode().

This PR implements the former. This allows a site to continue to allow paired browsing for unauthenticated users by adding plugin code that does add_filter( 'amp_dev_mode_enabled', '__return_true' );, though naturally they'd want to use some other condition (e.g. setting a cookie).

See #3365. Amends #3656.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).