Rails Cheatsheet: Add check of artifact storage space

What does this MR do?

Adds an artifacts size check to the Incorrect repository statistics shown in the GUI snippet, because the latter doesn't include the former.

Did I get that correctly, @rvzon? Do you agree with the refactoring I did to make the code a bit shorter?

---

@marcel.amirault: I cancelled the pipeline because I need to check something locally first. Please feel free to run it, if I haven't pushed a change by the start of your day, and Ron approved the snippet.

Related issues internal tickets

• https://gitlab.zendesk.com/agent/tickets/209800
• https://gitlab.zendesk.com/agent/tickets/200466

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