Properly deal with removed endpoints

!182 (closed) removed several endpoints using origin ids, and !995 (closed) removed some more endpoints related to origin types.

This caused an issue for some users, because one of them started returning 404 because the endpoint doesn't exist (while it used to mean the origin doesn't exist).

We should do something about it.

Possibilities:

1. Increment the API version number (probably 1.1, as they are relatively minor changes), and make all /api/1/ URLs return an error that is not a 404 (eg. 410 Gone)
2. Keep the current API version number but return an error (that is not a 404) for the endpoints we removed

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Migrated from T2158 (view on Phabricator)