the API should support redirection from one ID to another

[mhl]

This came up because of:

• https://github.com/mysociety/yournextmp-popit/issues/264

Any of the currently supported ways of merging people (either through the PopIt API's merge endpoint, or by using the PopIt API to update one person and delete the other) just delete one of the two people. This creates a problem from API consumers who don't have any way of telling whether a person who's suddenly 404ing has either just been deleted, or has been merged into someone else.

It would be great if:

• [ ] After merging people via the API's merge endpoint there was a 3xx redirection from the deleted person to the kept person
• [ ] There way in the API of creating such a redirection from an ID that would otherwise 404 (since YourNextMP has to do its own merging; the API merge endpoint won't do)

[akuckartz]

Linked data makes this easy using owl:sameAs.