Currently, some links aren’t properly built in Discobot when Discourse
is hosted in a subfolder. This is because we’re providing
`Discourse.base_url` to the Rails helpers which contains the base URL
*with* the prefix. But Rails helpers already handle this prefix so the
resulting link gets the prefix twice.
The fix is quite simple: use `Discourse.base_url_no_prefix` instead of
`Discourse.base_url`.