Reduce SQL timings in Projects::MergeRequestsController#ci_environments_status.json

The controller Projects::MergeRequestsController#ci_environments_status.json is one of our most frequently requested controllers with a 99th percentile (of SQL query timings) above 200 milliseconds. According to our daily overview the statistics are as follows:

• 401 000 requests per 12 hours
• 58.8% of the request time is spent in SQL

SQL timings per request:

• Mean: 50.93 ms
• 95th percentile: 136.97 ms
• 99th percentile: 717.06 ms

This controller should be improved so that the 99th percentile is consistently below 200 milliseconds, with an ideal target of 100 - 150 milliseconds.

TODO

• Remove the index yorick_deployments_test once our query improvements have been deployed