Show edit history of deleted users

[AntonKhorev]

Links to previous/next changesets from /changeset/<id>. Also links to /user/<name>/history, which becomes available for everyone. <name> is whatever the deletion procedure wrote to user's display name. This name is only used in urls, everywhere else the user is shown as "deleted".

Implements #1311

[tomhughes]

I think whether we can/should do this depends on us reaching a conclusion on how we can/should handle deleted accounts in general which in turn means deciding what, from a data protection point of view, we are allowed to do.

[AntonKhorev]

All that's made available here on the web interface is already available on the api.

[tomhughes]

Yes why we need to figure out the correct policy and apply it consistently everywhere - that might this PR or it might mean making the API match the web interface.

[AntonKhorev]

All changesets are available in dumps too.

[AntonKhorev]

Your current policy:
6c1d73a#diff-44438ce218f5287c58d0017f965d888715635d94280669896f75841fbd7b4cd7R245

All you can change is to make the edit history available only for registered users. But then that would likely apply to all history pages, not just ones of deleted users.