Refactoring Issues Board

## What does this MR do?

This MR aims to minimize conflicts between the CE issues board feature with EE multiple boards feature.

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

## Why was this MR needed?

To avoid a lot of conflicts with EE multiple boards feature.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [ ] ~~[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
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/929

https://gitlab.com/gitlab-org/gitlab-ee/issues/1084

See merge request !6727