Group blacklist

[GioBonvi]

I've been contacted privately by a user suggesting that it would be nice to be able to blacklist contacts en masse using contacts groups.

I found the idea interesting as it could be implemented in parallel to the already existing blacklisting functionality to offer two different levels of control: one easier to manage and broader in scope, the other more advanced and more precise.

I thought about implementing a "default" balcklisted group by having the blacklistedGroups array pre-populated as shown in the code example, but I fear it could lead to some confusion and add more problems than benefits.

  blacklistedGroups: ['GCEN-blacklist'],

Leaving the users to add their own groups adds a little bit of work for them, but should be simpler to understand.

Let me know what you think.