Content: Allow/Deny list for local network requests from the CMS

[dasgarner]

The CMS uses a HTTP client (curl/Guzzle) to fetch resources for inclusion into the CMS, examples of this include:

• Library: add via URL
• Widgets: any which require a URL, e.g. RSS ticker, Calendar
• DataSet: remote DataSets

It would be advantageous to implement a method for blocking access to locally hosted resources (e.g. localhost, private IPs). Many Xibo users rely on local network access, so introducing this must be on the basis of a non-breaking change for existing CMS instances.