Set new tabs to default to Tor Browser Home (about:tor) on desktop

I thought we already made this change in the past, however a recent fresh installation was still defaulting to "Blank Page".

Since about:tor hosts our research and fundraising efforts, it is preferable for it to be the default for new tabs too. This change is consistent with Firefox and Tor Browser for Android, both of which default to the "Home" page on new tabs.