Elasticsearch 6 support: migrate from parent/child relationships to joins

Elasticsearch 6 doesn't support creating new indexes with the _parent relationship. It will be removed altogether in ES7. The replacement is the join relationship.

Joins are supported in ES5 as well (after 5.6), so we should be able to move to it to support ES5-ES7.

It doesn't solve scalability / index size problems on large installations, but it's still worth doing as a (relatively) quick change while we work on #3217 (closed)

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Elasticsearch 6 was released in November. It has only a couple breaking changes - I'm not sure if any affect the features we use.

Let's set the specs to run against 6.0, do some manual testing, and see if we can mark it as supported or if extra work is necessary.

https://gitlab.zendesk.com/agent/tickets/85884

cc/ @nick.thomas @vsizov