Add chatops command for getting info about search migrations

Background

Currently, developers who wish to monitor Advanced Search migrations must look at Kibana Sidekiq logs and perform Elasticsearch queries against the production instance.

Proposal

Create a unified way for developers to get information on migration status.

1. Create an API to expose migrations - initial API endpoints: get all and get by version or name
2. Create chatops commands to interact with the API
   • chatops search migrations list
   • chatops search migrations get
   • chatops search migrations get