Add background migrations for Elasticsearch

What does this MR do?

This MR implements elastic migration framework, which looks and behaves similar to rails database migrations.

Migrations are stored in ee/elastic/migrate/ with YYYYMMDDHHMMSS_migration_name.rb file name for MigrationName class.

# frozen_string_literal: true

class MigrationName < Elastic::Migration
  def migrate
  end
end

Applied migrations are stored in gitlab-#{Rails.env}-migrations index. All unexecuted migrations are applied by Elastic::MigrationWorker cron worker sequentially.

New index example

❯ curl -s -H 'Content-type: application/json' -XPOST 'http://localhost:9200/gitlab-development-migrations/_search' -d '{}' | jq
{
  "took": 1,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 1,
      "relation": "eq"
    },
    "max_score": 1,
    "hits": [
      {
        "_index": "gitlab-development-migrations",
        "_type": "_doc",
        "_id": "20201105181100",
        "_score": 1,
        "_source": {
          "completed": true
        }
      }
    ]
  }
}

#234046 (closed)

Screenshots (strongly suggested)

Useful links

• rails/generators/migration.rb
• active_record/migration.rb

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #234046 (closed)