Multiple issue assignees(BE)

What does this MR do?

Implements multiple assignees for issues

Why was this MR needed?

Screenshots (if relevant)

Backend TODO:

• Implement a migration to create table
• Implement a migration to migrate an old data
• Fix ability policies for multiple assignees
• Check all the migrations on staging
• Notifications
• System notes
• https://gitlab.com/gitlab-org/gitlab-ee/tree/master/app/services/issuable_base_service.rb#L6
• https://gitlab.com/gitlab-org/gitlab-ee/tree/master/app/views/notify/_reassigned_issuable_email.text.erb#L6
• Implement counts cache for issues (update_assignee_cache_counts)
• Create an issue about removing issue.assignee_id field until next release.(https://gitlab.com/gitlab-org/gitlab-ce/issues/30453)
• Realations
• Unique index see Yorick's comment
• Split forms
• Chat commands
• Slash commands
• Fix for elasticsearch
• API v4
• API v3
• Final seacrh for issue\.assignee[^s] to make sure every obvious case is taken care.
• Issues export works
• Check out Importers
• Make sure DB seeds work
• Check issue board
• Check search (non-ES)
• Check filters
• Specs before applying new UI
• XML atom
• Make sure there is no N+1 problem
• TODOs
• Make sure CE has appropriate specs that normally would fail on EE only. (From Sean: however, I think it's really important that we can somehow test for this in CE, too. We really don't want to add a filter or a sort that only fails specs on EE)
• We should warn users about changed API and Web Hooks(https://gitlab.com/gitlab-org/gitlab-ee/issues/2365)
• Create an issue: "We could allow unassigning individual people here, like the existing label commands /label, /unlabel, and /relabel, but it doesn't need to happen now". (https://gitlab.com/gitlab-org/gitlab-ee/issues/2364)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/1904

Performance testing

Data to seed database https://gitlab.com/gitlab-org/gitlab-ce/snippets/1657660