Merge branch 'fix/get-cached-rendered-html-using-single-redis-request' into 'master'

Retrieve rendered HTML from cache in one request

## What does this MR do?
It consolidates requests made to cache store to check for cached HTML of multiple Markdown texts (for reference extracting purposes) into one request.

## Are there points in the code the reviewer needs to double check?
N/A

## Why was this MR needed?
To improve the performance of `Participable#raw_participants`

## What are the relevant issue numbers?
#19985 

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- ~~Tests~~
  - ~~[ ] Added for this feature/bug~~
  - ~~[ ] All builds are passing~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5406