Automate index alias update and old index deletion

See #191.

After indexing, update aliases to point to most recent indices and remove all but the two newest indices, if no other alias points to them (we don't want to remove production aliases when updating staging, and vice versa).

@dr0i I've tested this on quaoar2, and all seems good: http://quaoar2.hbz-nrw.de:9200/_plugin/head/. After this is merged, we should switch the automatic updates from weekends to a time where we can easily check for issues.

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