Whitelist entry trust level

[raffaeleflorio]

Add a trust level to whitelist entries in regard to other resources linked to the whitelisted one. Specifically the trust level is related to the other resources's host.
Three levels will be present:

• MIN (default). This means trust no other host. Implemented in ac58814 at L43.
• MID. Trust every resource loaded by every subdomain. Implicitly this is valid only for DNS name. Not yet implemented.
• MAX (covered in Add "originUrl trust" to the settings page #28). Trust every resource loaded by every host. Implemented in ac58814 at L44.

[raffaeleflorio]

This isn't implemented because it adds a lot of complexity and it's dangerous. Specifically there is the need of others trust level related to port, protocol and so on. For this reason this will be replaced by #34.