Reduce AutocompleteController#users call for reviewer dropdown from Sidebar and issuable form

See: Mar-25-2021_15-46-35

It seems to me that we're sending AutocompleteController#users requests too quickly and unnecessarily. I think we can reduce the number of request by simply increasing the delay after user's input.

Also, I think we can avoid prefilling initial dropdown list as they are often useless if you have more than 20 users.

This is the historical requests for AutocompleteController#users over the last 7 days

• Postgres slow query count: 14,758 - https://log.gprd.gitlab.net/app/discover#/?_g=h@daebe85&_a=h@45c7c37
• Rails request count: 3,275,620 - https://log.gprd.gitlab.net/goto/007bdb23f392231aa937116fc711d44d
• Slow query ratio: 0.45%

I think we could reduce the total request by half or more by doing this.