Spike: Establish backend tasks to support Epic dependencies

This issue is to scope out the engineering steps to deliver &2581 (closed). The expected outcome is a list of tasks that are estimated for capacity and will deliver a first iteration.

📅 Due Date: 2022-02-14

What we will build

• An exact replica of the current related issue widget, allowing epics to be related to each other.
• The ability to define the relationship as related, blocking or blocked-by.
• See #202431 (closed).

Additional Requirements

• As close as possible to the issues widget in design, as they will be merged into one when Epics become a Work Item type.
• Just as with issues, epics from different groups and hierarchies can be related by those who can interact with both. The user only sees what they have access to.

❓ Things to clarify

• ✅ Tier for this feature
  • GitLab Ultimate (See #351597 (comment 829659579))
• ✅ Solely REST in the first iteration or including GraphQL support (See slack thread (internal))
  • We can use solely Rest for an MVC (see #351597 (comment 836258525))
• ✅ Internal vs Public API
  • We will start with internal API plus one public endpoint to list related epics. (See #351597 (comment 832411238))
• ✅ Records storage - DB table name - columns needed - Model name.
  • Proposal to use related_epic_links name for table and Epic::RelatedEpicLink for model.
  • For columns we need 1-1 mapping of attributes/functionality with issue_links.
  • See See #351597 (comment 835799403) for details
• ✅ Backend services needed
  • Add new ListService, CreateService and DestroyService that inherit from IssuableLinks. (See #351597 (comment 835800236))
• ✅ Permissions performance impact
  • We need to check permissions for every epic but should not be too costly given the limited amount of related epics
  • We would get unavoidable N+1 queries for SELECT "saml_providers"...
  • See #351597 (comment 835834914) for details

📋 Tasks

1. Allow an epic to be marked as Related to, blocking or blocked by other Epics.

• Add a new table to store epic links #352437 (closed)
• Define permissions to create and view related epics #352439 (closed)

2. Allow to view, create, and delete related epics

• Add ability to query related epics of an epic #352495 (closed)
• Add services to support epic dependencies #352486 (closed)
• Add internal API endpoints to list, create, and delete related epics #352833 (closed)
• Add REST API endpoint to list related epics #352493 (closed)
• Add REST API endpoint to create and delete related epics #352840 (closed)