Feature/2735 - Require exclusive LDAP authentication and unchangeable permissions forms in production

[zrhoffman]

Resolves #2735. Feel free to change requirement descriptions/values as needed.

Because CU LDAP is only enabled in production, its hook_requirements() implementation does not check whether we are in production.

The Secure Permissions requirements check determins whether we are in production based on the contents of the express_core_version variable.

When not in production, the Express Permissions requirements check for secure permissions still runs, but its failure has only info-level severity, not error-level.