users: give all users unlimited favorites.

Let all users have unlimited favorites. Formerly the limit was 10k
favorites for regular members, 20k for Gold, and unlimited for Platinum.

Limiting favorites doesn't make sense since upvotes are unlimited.

test/unit/favorite_test.rb

@@ -47,12 +47,5 @@ class FavoriteTest < ActiveSupport::TestCase
       assert_equal("You have already favorited this post", error.message)
       assert_equal(1, @user1.favorite_count)
     end
-
-    should "not allow exceeding the user's favorite limit" do
-      @user1.stubs(:favorite_limit).returns(0)
-      error = assert_raises(Favorite::Error) { @p1.add_favorite!(@user1) }
-
-      assert_equal("You can only keep up to 0 favorites. Upgrade your account to save more.", error.message)
-    end
   end
 end