Replace /faq route in LMS urls

This route was mistakenly removed by the theming changes since the
`FAQ` marketing link actually points to `help_edx`, not `faq_edx`.

It would be good to rename the `FAQ` key in the `MKTG_URL_LINK_MAP` to `HELP` instead, since this would fix the issues we have in dynamically adding these marketing routes. That involves changing all instances of `marketing_link("FAQ")`, however.