Gray out Ads Panel for certain countries

[mandar-brave]

We will need a mechanism to prevent opt-in for Brave Ads for certain countries and provide an alternate grayed out Brave Rewards: Ads Panel experience.

List of countries will be provided offline and should be configurable.

@NejcZdovc

[mandar-brave]

@jenn-rhim when ready, update the issue with the right image.

[jsecretan]

The ads panel should be greyed out for all but: US, Canada, Germany, France, UK as those are the only places where we are launching ads to start.

[jenn-rhim]

Here is the version for the 'Ads unavailable in your region' case.

cc @mandar-brave

[srirambv]

QA Blocked on #3967 on Linux

[srirambv]

Verification passed on

Brave	0.63.46 Chromium: 74.0.3729.91 (Official Build) (64-bit)
Revision	03844ed83e02b8add3f4b9cb859a7108d55b2e4d-refs/branch-heads/3729@{#860}
OS	Linux

• Verified for Australia
  
• Verified for India (Kannada) for translated text. Sorry message translations pending
  

Verification passed on

Brave	0.63.46 Chromium: 74.0.3729.91 (Official Build) (64-bit)
Revision	03844ed83e02b8add3f4b9cb859a7108d55b2e4d-refs/branch-heads/3729@{#860}
OS	Windows 10 OS Build 17134.523

• Verified for India

• Verified for India (Hindi) for the translated text. Sorry message translations pending
  

Verified passed with

Brave	0.63.46 Chromium: 74.0.3729.91 (Official Build) (64-bit)
Revision	03844ed83e02b8add3f4b9cb859a7108d55b2e4d-refs/branch-heads/3729@{#860}
OS	Mac OS X

• Verified test plan from Disabling Ads Panel for disallowed regions brave-core#1941
• Verified did not see 'Sorry' message when IP and Region were set appropriately for included regions (US, Can, UK, FR, DE)
• Verified did see 'Sorry' message when IP and Region were set for non-included regions (Australia, Sweden, etc).