Special Routes should be upserted on startup

[KludgeKML]

(This is a transfer of the remaining part of issue https://github.com/alphagov/special-route-publisher/issues/260)

Publishing API now owns the rake tasks for publishing special routes, making it much easier to call than when they were in special route publisher - but it could be even easier if they were upserted on a deploy/app start, so that the process of adding or removing special routes became just merging them into the data file.

From the original issue (thanks @sengi )

We want these records to be pushed automatically (ideally on deployment pre-sync) by whichever component is supposed to own the information. I don't think we want this to continue to be a manually-initiated job, since that inevitably leads to production being out of sync with what's in the repo. Not to mention yet another Ruby app to maintain with the dependency-update toil that involves.