Only expand `{{...}}` in messages once (part 2)

Only expand `{{...}}` in messages once (part 2)

If we're going to call `OutputPage::addWikiTextAs*` or
`OutputPage::parseAs*` to parse the message, we don't need to
pre-expand `{{...}}` markup before passing it to the parser.

Follow up to I2f68f73365df876e6f9d017e4e3ddea39c4a1284.

Bug: T206574
Change-Id: I0ee493207ea80c1caba7e60a31714805dc54c64e