Add en-US link for the support portal

The support portal has no explicit en-US locale in the URL or the permalinks, for example https://support.torproject.org/tbb/tbb-2/ (compare this with https://support.torproject.org/de/tbb/tbb-2/ for Deutsch).

Would it be possible to add this explicitly? This is helpful for #27558 (moved) and will match the URL of the Tor Browser English manual https://tb-manual.torproject.org/en-US/.

So,

https://support.torproject.org/tbb/tbb-2/

should instead be:

https://support.torproject.org/en-US/tbb/tbb-2/