Add rake task to seed reservations from panopticon

This will allow us to switch th using url-arbiter as the source of truth
for which app owns a given path.
Request the minimal artefact view from Panopticon.

Once <https://github.com/alphagov/panopticon/pull/196> is merged and
deployed, this will speed up the seeding significantly; until that time,
it won't have any effect.
Merge pull request #7 from alphagov/panopticon_importer

Add rake task to seed reservations from panopticon