TB 42738: Move override logic to the about:tor commit.

TB 42738: Temporarily revert the about:tor changes to
BrowserContentHandler.sys.mjs.

TB 43929: Update about:tor override page logic for ESR 140.

Unlike ESR 128, we do not try and move the startPage logic to earlier in
the method since it would complicate the patch and upstream's logic.
Instead, we delay out duplicate about:tor logic until the end of the
method.