WIP: Use artifacts_size to check for build artifacts existence

What does this MR do?

It uses Ci::Build#artifacts_size instead of doing actual artifact file existence. Testing on this repo's pipelines index, request time was reduced from ~8 seconds to ~3 seconds.

Are there points in the code the reviewer needs to double check?

The introduction of nullifying artifacts_size is pretty recent (around the end of last June), do we need a migration for old records?

Why was this MR needed?

To improve the speed of pages that depend on Ci::Build#artifacts? extensively (like pipelines index).

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together