Optimize "cache_key" using a concern
## What does this MR do?
This MR adds a concern (used by Issue and Note) that provides an optimized version of Rails' `cache_key` method. See 77c8520e for more details.
## Are there points in the code the reviewer needs to double check?
No, though a spell check would be appreciated.
## Why was this MR needed?
When loading a lot of data from Redis (e.g. an issue with lots of notes) quite a large amount of time is spent in generating cache keys. This is due to multiple reasons such as:
* Rails trying to figure out if it should use `updated_at` or `updated_on` using somewhat inefficient code
* Rails relying on pluralization logic to figure out how to generate a cache namespace using a model name
* Rails calling a whole bunch of methods in general in the process of generating cache keys
In short, Rails is trying to cater to every possible use case, at the cost of performance.
## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/13651 is not directly related but I ran into this `cache_key` problem when looking into said issue.
See merge request !5715
