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 

## Screenshots (if relevant)
![ScreenShot-P216](/uploads/1af5e71a0a0ff0a60c5d7b54c0e09d9c/ScreenShot-P216.png)

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

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] 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 !5725