Use an actual URL for helppage in enotif_body

The helppage can be a full URL, which canonicalurl doesn't like.

This means that if a wiki
* has a custom enotif_body which uses canonicalurl + helppage, but
* doesn't have helppage defined locally (as local/interwiki title),
then it will still have to fix either of the two locally.

Bug: 63269
Change-Id: I82d26b648ef23b69ae4fbd0a74ffa23eb7a93dae