Ability to bulk assign issues to author of merge request

What does this MR do?

Provides a link to auto-assign issues to the author of a merge request, when they are mentioned as being closed by the MR.

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

Why was this MR needed?

To help avoid working on a MR without having assigned related issues to self

What are the relevant issue numbers?

Fixes #18876 (closed)

Screenshots (if relevant)

Tasks

• Refactor or move away from using BulkUpdateService
• Consider alternate link message when only a subset of issues will be assigned
• Minimize repeated calls to expensive closes_issues method
• Move away from using inflector for pluralization and fix flash message
• Change auth before_action and fallback to error flash message
• Shouldn't overwrite current assignee if one exists

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