We’ve seen a deadlock in CI here https://gitlab.com/mkozono/gitlab-ce/builds/15644492#down-build-trace. This commit should not fix that particular failure, but perhaps it will avoid others.

* Don’t call delete_conflicting_redirects after update if the path wasn’t changed
* Rename descendants without using recursion again, so we can run delete_conflicting_redirects exactly once.

Note: This changes the behavior of user lookups (see the spec change) so it acts the same way as groups and projects. Unauthenticated clients attempting to access a user page will be redirected to login whether the user exists and is publicly restricted, or does not exist at all.

Not sure why this fixes this error:

```
ActiveRecord::RecordNotUnique:
       PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_routes_on_source_type_and_source_id"
       DETAIL:  Key (source_type, source_id)=(Project, 1) already exists.
       : INSERT INTO "routes" ("source_type", "path", "name", "source_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"
```

User who is not allowed to trigger manual actions should not be
allowed to reprocess / retrigger / retry these actions.