Update elasticsearch to the latest version (7.0)

[ppolischuk]

On staging, we're using ES version 6.2.4. In Docker, we're running 6.2.3. The latest version of ES is 7.0

We need to run comprehensive tests to see if anything breaks.

Breaking changes for ES version 7.0: https://www.elastic.co/guide/en/elasticsearch/reference/7.0/breaking-changes-7.0.html

Joe read the above doc and found the following bits relevant:

• Discovery configuration is required in production - Need to keep an eye on this. See Event Data config for reference, but we may need to re-jig it.
• _flush and _force_merge will no longer refresh - May impact tests I wrote.
• The _all meta field is removed - May impact tests. Let's try it.
• Limiting the number of nested json objects - May impact funding info?
• hits.total is now an object in the search response - Try it.
• Internal Versioning is no longer supported for optimistic concurrency control - May actually be an issue for Event Data.

[ppolischuk]

Joe and Esha decided not to pursue this right now for the migration.