Older sites can end up with duplicate rows in the
blocktype_externalfeed_data for the same url. This causes problems if
the site is using latest mahara as it stops one being able to add
/edit a feed if there are duplicate rows for that feed's url.

Change-Id: I47ee3952dbe75b5643adf55105a9219a3e3e8cd9
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Signed-off-by: Aaron Wells <aaronw@catalyst.net.nz>