Let users decide whether to load their home page on new identity.

In #41765 (closed) the custom home page was disabled due to a very low-security issue. However, this has created a number of complaints from users.

Thus this issue to implement a way to deal with both cases.

Options on the table are adding a setting/pref to restore old behavior, display a chrome banner with a string explaining why its not loaded, with a button to load it anyways; finally a combination of the two: add a setting/pref to restore the old behavior and display a chrome banner with a short explanation about why it is not loaded, as well as instructions for how to turn that behavior back on