Add bulk update support for merge requests list

What does this MR do?

Adds new feature: ability to update multiple merge requests at once from index page (same like we have for issues already)

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

I don't think so

Why was this MR needed?

• consistency with issues
• save user time when you want set same label/milestone to 20 merge requests

Screenshots (if relevant)

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20087