Refactor Gitlab::Identifier

## What does this MR do?

This refactors `Gitlab::Identifier` so that it:

1. Has tests
2. Caches output in an instance variable to reduce queries
3. Uses only a single query to find a user by an SSH key, instead of 2

## Why was this MR needed?

This code was untested and would execute more SQL queries than needed.

See merge request !6680