Introduce a BlockManager service

Introduce a BlockManager service

This introduces a minimal BlockManager service, for getting blocks
that apply to a User.

Move the part of User::getBlockedStatus that checks for the blocks
into BlockManager::getUserBlock, and move the related helper
methods from User to BlockManager.

Hard deprecate or remove these helper methods, and move to private
methods in the BlockManager:
* User::getBlockFromCookieValue
* User::isLocallyBlockedProxy
* User::inDnsBlacklist

Soft deprecate these helper methods, and move to public methods in
the BlockManager:
* User::isDnsBlacklisted

Add tests to cover the methods moved to BlockManager.

Bug: T219441
Change-Id: I0af658d71288376735cebe541215383b56bb72e5