Elasticsearch 7 compatibility

[notbakaneko]

The form of some of the queries currently being used won't work with ES7+ (especially the index_meta ones).
This metadata should probably be moved into the high_score indices themselves; the metadata queries can be replaced by listing and filtering for matching indices.

[peppy]

Keep in mind we're also switching to a new storage format which is probably going to require a lot of changes here, so I wouldn't make any changes before that happens (will give more details in the next week or two).